UNIVERSITY OF OKLAHOMA
GRADUATE COLLEGE
AN INVESTIGATION OF SUSTAINABLE FLUORIDE WATER TREATMENT
TECHNOLOGIES WITH A FOCUS ON ETHIOPIA
A DISSERTATION
SUBMITTED TO THE GRADUATE FACULTY
in partial fulfillment of the requirements for the
Degree of
DOCTOR OF PHILOSOPHY
By
LAURA R. BRUNSON
Norman, Oklahoma
2014
AN INVESTIGATION OF SUSTAINABLE FLUORIDE WATER TREATMENT
TECHNOLOGIES WITH A FOCUS ON ETHIOPIA
A DISSERTATION APPROVED FOR THE
SCHOOL OF CIVIL ENGINEERING AND ENVIRONMENTAL SCIENCE
BY
__________________________________
Dr. David A. Sabatini, Chair
__________________________________
Dr. Lowell Busenitz
__________________________________
Dr. Elizabeth C. Butler
__________________________________
Dr. Robert C. Knox
__________________________________
Dr. Robert Nairn
© Copyright by LAURA R. BRUNSON 2014
All Rights Reserved.
This dissertation is dedicated to the women and children around the world that spend
countless hours each day walking to collect water for their families.
Acknowledgements
None of this work would have been possible without the advice, support,
encouragement, teaching, and mentoring of Dr. David Sabatini. Dr. Sabatini has served
as my graduate adviser for over eight years. He encouraged me to come to OU for my
graduate work and to pursue graduate degrees in Environmental Science even though I
did not come from a science or engineering background. From Dr. Sabatini, I have
learned technical aspects about engineering and science, as well as writing, leadership,
and how to be a better human being. I will utilize the lessons I have learned from Dr.
Sabatini throughout the rest of my life.
I would also like to thank Mrs. Frances Sabatini who has been a constant cheerleader
throughout my graduate school experience. Thanks to Peggy Sabatini, who has been a
graduate assistant in the lab for many years, and after our travels to Ethiopia, is like a
little sister to me.
Peggy and Frances have both provided countless laughs and
encouragement.
Thank you to the additional committee members for this dissertation – Dr. Elizabeth
Butler, Dr. Lowell Busenitz, Dr. Robert Knox, and Dr. Robert Nairn. These four
faculty members have spent hours teaching me, both in their classes and through
additional questions and paper reviews. I thank them for their time and guidance
towards making the research stronger and making this a better dissertation.
I thank Dr. Andy Madden who trusted me to spend many hours in his lab using several
pieces of characterization and measurement equipment. I thank him for his time
teaching me how to use them. Thanks to Dr. Nairn and Dr. Zaman for the use of their
microwave digestion equipment and high temperature oven, respectively.
I am very grateful for funding from the following sources: the Sun Oil Company
Endowed Chair, the National Science Foundation Graduate Fellowship Program, the
iv
Environmental Projection Agency Science To Achieve Results Fellowship Program, the
OU WaTER Center, the Graduate College at the University of Oklahoma, and the
College of Engineering at the University of Oklahoma.
I appreciate the helpful and generous assistance with funding paperwork, enrollment,
logistics and much more from Molly Smith, Brenda Clouse, Susan Williams and Audre
Carter in the College of Engineering.
The work done in rural Ethiopia could not have been done without the generous help of
Catholic Relief Services Ethiopia, the Meki Catholic Secretariat, the Oromo Self Help
Organization, CARE Ethiopia, Dr. Feleke Zewge, Teshome Lemma, Esayas Samual,
Bekele Abaire, and most of all, Meseret Desalegn who was with us every step of the
way during the several months of continuous flow column studies.
Many thanks to the undergraduate lab assistants who went above and beyond providing
help with lab work which included endless washing of glassware and batch test bottles.
I thank them also for their friendship and entertainment. Over the years my wonderful
lab assistants were Peggy Sabatini, Tessa Blanchard, Erik Tichansky, Aislinn McLeod,
Katie Heitman, and Peter Everest.
I am so appreciative of the support and encouragement and teaching that came from lab
mates over the years. I learned so much from all of them in various ways about
research and writing, lab equipment, and how to do posters and research presentations.
Thanks to “Oat” Witthayapanyanon, Tri Pham, Linh Do, “Mink” Attaphong, Chris
Cope, Hayley Ryckman, Damon Webster, Teshome Lemma, Junyi Du, Sezin Kadioglu,
Anisha Nijhawan and others that came to visit for a short time, for the learning, fun and
friendship-building times in the lab.
In great part, this PhD was made possible by the friends and family who prayed for me
through presentations, papers, exams, and the final defense. I would like to specifically
thank Virginia Tackett and Elda Shafer who were grandmas that weren’t here to see this
v
dissertation come to a conclusion, but who provided numerous prayers during the years
when they were still here. To my “Carla Mom”, Carla Walker who provides such
thoughtful support, I thank you.
It is impossible to adequately thank the many friends and family that were with me
throughout some or all of my graduate work. These loved ones provided laughs and
fun, put up with my erratic schedule and lack of socializing, invited me for homecooked meals, shared their homes and sometimes their kids (“Auntee Laura’s kids”),
made me take research breaks, commiserated during failures or frustrations, celebrated
successes, encouraged me, shared their lives with me and overall were there for me day
and in day out for years. I am deeply grateful for the love, support and friendship of
this wonderful group of people and I am lucky to have them in my life.
vi
Table of Contents
Page
ACKNOWLEDGEMENTS.............................................................................................iv
TABLE OF CONTENTS………………………………………………………………vii
LIST OF TABLES…………………………………………………………………........x
LIST OF FIGURES…………………………………………………………………….xii
ABSTRACT…………………………………………………………………………...xvi
CHAPTER 1 Introduction…………………………………………………………...…1
Safe Drinking Water and Fluoride………………………………………………1
Fluoride Removal Technologies…………………………………………………2
Goal and Objectives……………………………………………………………..7
Overview of Chapters……………………………………………………………8
References……………………………………………………………………...10
CHAPTER 2 In Pursuit of Sustainable Water Solutions in Emerging Regions………16
Abstract………………………………………………………………………....16
Introduction………………………………………………………………….....16
Implementation Challenges…………………………………………….............17
Sustainable Development………………………………………………………21
Social and Cultural Assessment………………………………………………..23
Market-based Implementation……………………………………………….....26
Technology Development……………………………………………………...31
Sustainability Tools………………………………………………………….....32
Practical Implications………………………………………………………......33
vii
Conclusion……………………………………………………………………...34
Acknowledgements………………………………………………………….....35
References……………………………………………………………………...35
CHAPTER 3 The Role of Surface Area and Surface Chemistry during an
Investigation of Eucalyptus Wood Char for Fluoride Adsorption from
Drinking Water…………………………………………………………………………43
Abstract…………………………………………………………………………43
Background……………………………………………………………………..44
Materials and Methods…………………………………………………………49
Results and Discussion…………………………………………………………52
Conclusion……………………………………………………………………...64
Acknowledgements…………………………………………………………….65
References……………………………………………………………………...66
CHAPTER 4 Practical Considerations, Column Studies and Natural Organic
Material Competition for Fluoride Removal with Bone Char and Aluminum
Amended Materials in the Main Ethiopian Rift Valley.……..…………………………72
Abstract…………………………………………………………………………72
Introduction…………………………………………………………………….73
Methods………………………………………………………………………...78
Results and Discussion…………………………………………………………81
Conclusion……………………………………………………………………...95
Acknowledgements…………………………………………………………….96
References……………………………………………………………………...96
viii
CHAPTER 5 Methods for Optimizing Activated Materials for Fluoride Removal
from Drinking Water Sources………………………………………………………...104
Abstract……………………………………………………………………….104
Introduction...…………………………………………………………………105
Materials and Methods………………………………………………………..110
Results and Discussion………………………………………………………..114
Conclusion…………………………………………………………………….135
Acknowledgements…………………………………………………………...136
References…………………………………………………………………….137
CHAPTER 6
Conclusions..………………………………………………………………….142
APPENDICES
Appendix A: Fluoride Removal by Bone and Wood Chars…………………………..149
Appendix B: Competing Ions…………………………………………………………151
Appendix C: Iron Oxide Amendments………………………………………………..152
ix
List of Tables
Page
2.1
Examples of water projects in developing countries and possible
improvements that could increase sustainability. Text in grey indicates
projects with a high level of success………………………………………..19-20
2.2
Tasks that fall into each component of synergistic research towards
developing
sustainable
research
and
water
access
or
treatment
implementations………………………………………………………………..23
3.1
Freundlich constants calculated from batch tests investigating fluoride
removal of aluminum amended and unamended wood and bone chars………..54
3.2
Specific surface area and pHPZC of aluminum amended, reducing agent
modified and unamended eucalyptus wood and bone chars at various
charring temperatures………………………………………………………..…56
3.3
Freundlich constants calculated from batch tests investigating fluoride
removal of bone and wood chars modified with reducing agents and
amended with aluminum or unamended. In all cases 1/n values have been
forced to a common values so Kf constants can be statistically compared…….61
4.1
Qe data for six media evaluated in batch isotherm studies……………………..82
4.2
Qe in mg of fluoride removed per gram of material calculated based on the
amount of fluoride removed during the continuous flow column study run
to saturation (Ceffluent = Cinfluent). The bed volumes to Ceff = 1.5 mg/L
obtained using the data presented in 4.1. Using the Langmuir equation
both the Langmuir Qm and the Qe value at 8.6 equilibrium fluoride
concentration are presented…………………………………………………….86
x
4.3
Results of batch isotherms studying fluoride removal with and without
the presence of sulfate and natural organic material in solution……………….93
5.1
Freundlich and Langmuir Constants and Qe1.5 data obtained using
batch isotherm data for wood char alone and metal oxide amended
wood char……………………………………………………………………..116
5.2
Freundlich and Langmuir Constants obtained using batch isotherm
data for untreated, amended and pretreated wood chars……………………...119
5.3
Metal loading, specific surface area and surface chemistry characteristics
of all materials tested………………………………………………………….124
C-1: Results of preliminary iron oxide amendment tests conducted on wood char with
variations in amendment time, concentration and method…..……..…………152
xi
List of Figures
Page
2.1
Diagram showing the three discussed areas crucial to successful
implementation, their overlap where a sustainable outcome should be
obtained, and their embedding in a broader systems perspective (the
enabling environment)………………………………………………………….22
3.1
The equilibrium fluoride concentration, Ce, in mg/L (x-axis) versus mg
of fluoride removed per gram of material, Qe, (y-axis) for both aluminum
amended and unamended eucalyptus wood chars……………………………...55
3.2
Bar graph shows specific surface area (m2/g) (left y-axis) and line graph
shows the pHPZC (right y-axis) while charring temperature for both
aluminum amended and unamended eucalyptus wood chars is shown
on the x-axis.…………………………………………………………………...57
4.1
Results of continuous flow columns studies where the x-axis shows bed
volumes and the y-axis shows effluent fluoride concentrations in mg/L.
4.1A. Lab studies where the influent pH (unadjusted) was approximately 6.0 and
the solution was fluoride in deionized water. 4.1B. Field studies using native
groundwater with an influent pH of approximately 8.2 ......………………83-84
4.2
Graph shows results of continuous flow columns studies where the x-axis
shows bed volumes and the Y axis shows effluent fluoride concentration in
mg/L. Influent fluoride concentration was 8.6 mg/L and pH and competing
ions vary as stated in the key…………………………………………………...89
4.3
Graph shows results of rapid small scale column tests where the x-axis
shows bed volumes and the y-axis shows effluent fluoride concentration
xii
in mg/L. Columns have volumes of 34.9 and 7.9 cm3 and average particle
sizes of 637 and 302 μm , for large and small scale respectively. Starting
concentration and flow rate were equivalent for both at 8.6 mg/L fluoride
and 1.2 ml/min…………………………………………………………………94
5.1
Results of batch isotherms showing the fluoride removal capacities of
untreated wood char and metal oxide amended wood chars. Qe, the mg of
fluoride removed per gram of media is plotted against Ce, the equilibrium
concentration of fluoride in mg/L……………………………………………..115
5.2
Results of batch isotherms showing the fluoride removal capacities of
untreated wood char and metal oxide amended wood chars with and
without pretreatment (KM = potassium permanganate, HP = hydrogen
peroxide). Figure 5.2A shows results of wood char treated with aluminum
sulfate and Figure 5.2B shows results of wood char treated with iron (III)
chloride. Qe, the mg of fluoride removed per gram of media is plotted
against Ce, the equilibrium concentration of fluoride in mg/L…………..117-118
5.3
Results of material analysis where 5.3A shows metal loading percentages,
5.3B shows point of zero charge values and 5.3C shows specific surface
area data. Numbers above the bars in each graph note the Qe1.5 values for
that media…………………………………………………………...……122-123
5.4
Data shows the fluoride removal capacities of untreated wood char and
metal oxide amended wood chars (WC), with and without pretreatment,
(KM = potassium permanganate, HP = hydrogen peroxide) normalized by
metal content. 5.4A shows results of wood char treated with aluminum
xiii
sulfate, 5.4B shows wood char treated with aluminum chloride, 5.4C
shows wood char treated with iron (III) nitrate with the inset graph showing
the data only up to Ce = 5 and 5.4D shows wood char treated with
iron (II) chloride. The y-axis shows, Qe/metal %, the mg of fluoride removed
per gram of media normalized by metal content, and is plotted against Ce,
the equilibrium concentration of fluoride in mg/L………………..……...127-129
5.5
Results of batch isotherms showing the fluoride removal capacities of
untreated wood char and metal oxide amended wood chars normalized by
both specific surface area and metal content. Qe, the mg of fluoride
removed per gram of media is normalized by specific surface area
and metals content (mg fluoride * g char/g metal * m2) and plotted
against Ce, the equilibrium concentration of fluoride in mg/L………………..134
A-1
Graph shows adsorption isotherms of fluoride removal by Ximenia
americana (XA) and Eucalyptus robusta (EU) wood chars amended
with aluminum oxides, plotting fluoride levels adsorbed by the chars
(Qe, mg fluoride/g wood char) versus equilibrated aqueous
fluoride concentration (Ce, mg/L)………………………………………….…149
A-2
Graph shows adsorption isotherms of fluoride removal by fish and
bone chars and aluminum oxide amended fish and bone chars, plotting
fluoride levels adsorbed by the chars (Qe, mg fluoride/g wood char)
versus equilibrated aqueous fluoride concentration (Ce, mg/L)……………....150
B-1
Adsorption isotherms of fluoride on fish bone char (FBC), with
competing ions sulfate (SO4) and phosphate (PO4) present in some
xiv
systems as noted in the legend. The y-axis shows fluoride levels
adsorbed by the fish bone char (Qe, mg fluoride/g fish bone char) and
the x-axis shows equilibrated aqueous fluoride concentration (Ce, mg/L)…....151
.
xv
Abstract
Human consumption of unsafe drinking water from an unimproved source is a
global issue affecting approximately 748 million people worldwide. While this number
has been decreasing in recent years, an additional 1.2 billion people are estimated to
lack access to water that is consistently free from health risks. This dissertation begins
with a literature review investigating drinking water improvement initiatives around the
world and a discussion of reasons why these initiatives often fail. Resources are
provided for researchers and practitioners working on drinking water treatment
implementations and examples of implementations that have failed or succeeded are
discussed. The conclusion from this review is that global drinking water solutions will
be more effective when designed and implemented by personnel from multiple
disciplines.
For example, people in several fields, including: social sciences,
engineering and business, should collaborate and share ideas and expertise. Ideally this
collaboration should start at the genesis of a project and continue through
implementation and follow up.
There is hope that the synergistic efforts of
multidisciplinary teams will help to increase the number of successful water initiatives.
Next the dissertation focuses on the problem of elevated fluoride concentrations
in drinking water. Naturally occurring fluoride is the second largest issue contributing
to the global water crisis. It is estimated that globally over 200 million people are
affected by elevated concentrations of fluoride in drinking water. The goal of the
technical portion of this dissertation is to investigate locally available and sustainable
materials that can be used to remove fluoride from drinking water, with a focus on
Ethiopia and eastern Sub-Saharan Africa. Bone char is very effective as an adsorptive
xvi
material, but is not always accepted by communities due to religious or cultural beliefs.
Therefore, this research evaluated methods to improve the fluoride removal capacity of
bone char as well as investigated materials that might serve as a replacement for bone
char in appropriate communities. Eucalyptus trees are prevalent in Ethiopia where a
large fluoride problem exists, and thus, eucalyptus wood char was investigated as a
potential substitute for bone char. This dissertation studied wood char produced from
Eucalyptus robusta as an adsorption material to remove fluoride from water, thereby
making it safe for consumption. Although the use of eucalyptus wood char alone
removed minimal fluoride, when it was amended with aluminum and iron oxides it
evidenced much higher fluoride removal capacities. Metal oxides, produced from
starting materials such as aluminum sulfate and iron (III) nitrate, were used to amend
the wood char. Metal amendments resulted in fluoride removal capacities ranging from
3 to 50 times higher than wood char without amendment. The combination of wood
char and metal oxide amendment is synergistic because the wood char provides a matrix
with a high specific surface area for the metal oxides to adhere to while the metal oxide
amendment increases the electrostatic attraction of the char surface for fluoride.
Additionally, wood char was pretreated with oxidizing agents such as hydrogen
peroxide and potassium permanganate prior to metal amendment. These pretreated and
metal amended chars were found, in most cases, to have increased metal loading rates
and, in some cases, higher fluoride removal capacities.
This dissertation also looked at ways to improve the adsorption effectiveness of
bone char for communities where bone char is an acceptable material for drinking water
treatment. Amending bone char with aluminum nitrate showed an increase in fluoride
xvii
removal at high equilibrium concentrations but not at low equilibrium concentrations
close to the WHO recommended value for fluoride (1.5 mg/L). Select fluoride removal
materials, including aluminum impregnated wood char, activated alumina, bone char
and aluminum amended bone char, were studied in the field using groundwater from a
well in a rural Ethiopian community. Field results suggested that the combination of
elevated groundwater pH along with competing ions such as sulfate (both common in
ground waters of the Ethiopian Rift Valley) affected the fluoride removal capacities of
the materials studied, particularly the aluminum impregnated wood char. Finally, this
research tested the validity of the Rapid Small Scale Column Tests (RSSCT) principles
for bone char adsorbing fluoride from water. These experiments indicated that RSSCT
principles are applicable for bone char; use of this approach can result in large time and
cost savings to researchers and implementers. Overall, this dissertation provides several
conclusions that are practically helpful to researchers in the field and also foundational
research on which future studies can build to continue efforts to find sustainable and
appropriate fluoride removal technologies.
xviii
CHAPTER 1
Introduction
Safe Drinking Water and Fluoride
Lack of safe drinking water is a key global issue. The World Health
Organization (WHO) and the United Nations International Children’s Emergency Fund
(UNICEF) estimate that 748 million people currently lack access to an improved water
source (WHO and UNICEF, 2014). Onda et al. (2012) suggested that in addition to the
748 million people without an improved water source, another 1.2 billion people may
be consuming water from sources that are improved but still at risk for microbial
contamination. Lack of safe water is further exacerbated by the reality that 1.4 billion
people are estimated to live on less than $1.25 (US) per day (Chen and Ravallion,
2008). In addition, water-related health issues contribute to concerns such as lack of
education, gender equity and economic development. To mitigate these drinking water
issues, sustainable, inexpensive and locally available water treatment technologies must
be developed, improved and implemented.
After pathogens, the largest issue contributing to lack of safe drinking water is
naturally occurring fluoride, which is estimated to affect 200 million people globally
(Amini et al., 2008). The WHO recommended fluoride limit in drinking water is 1.5
mg/L (World Health Organization, 2011). Fluoride is naturally occurring in the
groundwater of many areas of the world including: the Rift Valley of Africa, portions of
China and India and parts of the southwestern United States (Brindha et al., 2010;
Fewtrell et al., 2006; Meenakshi and Maheshwari, 2006; Nigussie et al., 2007).
1
Although fluoride is naturally occurring in groundwater, it can also occur due to
anthropogenic processes such as aluminum smelting and fertilizer production (Hem,
1985; Hudak, 2008). Fluoride is an example of an element where the dose consumed
determines whether it is beneficial or toxic. In low doses fluoride helps to prevent
dental carries and, thus, improves the health of teeth (Fawell et al., 2006; Schamschula
and Barmes, 1981). However, consumption of higher concentrations can be harmful as
it causes dental and skeletal fluorosis (Fawell et al., 2006; Gazzano et al., 2010). Dental
fluorosis results in darkening or mottling of teeth, which, while not physically
debilitating, can cause social and financial issues due to the stigma of mottled teeth.
Skeletal fluorosis can cause bones to become deformed or stiff which can limit mobility
and/or cause pain (Kaseva, 2006; Meenakshi and Maheshwari, 2006). Therefore, many
researchers are investigating fluoride removal technologies (Abe et al., 2004; Ayoob et
al., 2008; Gwala et al., 2011; Nigussie et al., 2007).
Fluoride Removal Technologies
Water treatment methods frequently investigated for fluoride removal include:
precipitation, ion exchange, electrocoagulation, and adsorption (Brunson and Sabatini,
2009; Gwala et al., 2011; Kamble et al., 2007; Meenakshi and Maheshwari, 2006; Rao,
2003; Tchomgui-Kamga et al., 2010). For rural regions in developing countries,
techniques should be inexpensive, sustainable and use locally available materials for
treating water containing high concentrations of fluoride. The Nalgonda technique
utilizes alum and lime to produce aluminum oxide precipitates that sorb and settle
fluoride ions. This technique is popular because of its low cost and use of chemicals that
2
are easily obtained. However, the Nalgonda technique requires frequent addition of
chemicals, produces copious amounts of waste sludge, and is not efficient for fluoride
removal when the starting concentration is high (Fawell et al., 2006; Nigussie et al.,
2007). While electrocoagulation can effectively remove fluoride, it requires some form
of electricity (Gwala et al., 2011). Activated alumina is an adsorptive material
frequently used in filtration systems. Adsorption can be helpful for fluoride removal
because it operates using gravity flow, and, depending on the sorptive material, can be
effective at treating water to meet the WHO fluoride standard. Activated alumina can be
effective, but is a manufactured material that may be prohibitively expensive to obtain
in rural developing regions. Therefore, research evaluating less expensive and locally
available adsorptive materials is desirable in an effort to respond to the limitations of
techniques discussed above.
Bone char has been widely investigated as an adsorptive material for removal of
fluoride and several trace metals (Bhargava and Killedar, 1991; Kaseva, 2006; Larsen et
al., 1994; Thomson et al., 2003). Bone char is primarily composed of hydroxyapatite
(70-76%) and carbon (8-11%) and is currently produced in several developing countries
from waste bones, which makes it relatively inexpensive (Medellin-Castillo et al., 2007;
Müller, 2007; Purevsuren et al., 2004; Wilson et al., 2003). Charring animal bones
removes organic matter from the bone structure, producing a large specific surface area
which is helpful for high adsorption capacity (Chidambaram et al., 2004; Kaseva,
2006). For example, Medellin-Castillo et al. (2007) reported a specific surface area of
104 m2/g for their tested bone char. This is similar to levels reported by others, such as
150 m2/g (Chen et al., 2008) and 105 m2/g (Brunson and Sabatini, 2009). Variations in
3
specific surface area can be attributed to char source, type of bones and preparation
method. Bone char also exhibits favorable surface chemistry with Medellin-Castillo et
al. (2007) and Brunson and Sabatini (2009) both finding the point of zero charge
(pHPZC) of bone char to be close to 8.3. In solutions with pH values below the pHPZC, the
bone char surface will exhibit a positive charge, while at solution pH values above the
pHPZC the material will exhibit a negative charge. Thus, for fluoride removal occurring
in groundwater with near neutral pH values, a material with a high pHPZC will evidence
a positive surface charge which will attract negatively charged fluoride ions. Despite its
favorable surface chemistry and effective fluoride removal, bone char is not accepted in
some communities due to religious and cultural reasons or poor quality of char
production (Fawell et al., 2006), leading to the need for alternate materials.
New research investigating the potential for using a carbon-based material, such
as wood char, as an adsorption material is helpful for locations where bone char is not
accepted. Wood is typically composed of carbon (50%), oxygen (44%), hydrogen (6%)
and trace amounts of metals (Pettersen, 1984; Rowe and Conner, 1979). However, the
chemical composition and other characteristics of wood can vary based on location, soil
type, season, age and type of wood (Pettersen, 1984). The specific surface area of wood
char is shown to differ greatly depending on charring conditions, wood type and
location of wood growth. For example, James et al. (2005) showed wide ranges of
specific surface area for wood chars produced in laboratory conditions, ranging from
1.0 to 397 m2/g, and different kinds of wood charred in nature, ranging from 1.7 to 85
m2/g. Although wood char in some cases exhibits a high surface area, a study by Abe et
al. (2004) showed that bone char has a much higher fluoride removal capacity than
4
several tested wood chars. Therefore, methods are needed to improve the fluoride
removal capacity of wood chars.
Metal amendments have shown promise as one way to improve the fluoride
removal capacity of wood chars. For example, studies have shown promising results for
removing fluoride with aluminum or iron-based precipitates.
Levya-Ramos et al.
(1999) found that impregnating activated carbon with aluminum greatly improved the
fluoride removal capacity. A material produced from aluminum and iron was studied
by Biswas et al. (2007) and results showed a maximum removal value of 18 mg of
fluoride removed per g of material as calculated by the Langmuir equation. The success
of fluoride adsorption by metal amendments is attributed to the attraction between the
negatively charged fluoride ions and the positively charged metal oxide surfaces,
resulting in ion exchange between fluoride and hydroxide ions on the material surface.
While bone char has previously shown a high fluoride removal capacity, it is not
reaching the removal effectiveness of some of the aluminum/iron based materials
discussed above, which means it may be useful to investigate ways of enhancing
fluoride uptake with bone char.
A final approach of interest is pretreating charred media with either oxidizing or
reducing agents to alter their surface chemistry. Pretreatment methods could be
invaluable for their effects on surface chemistry. Polovina et al. (1997) suggested this is
one of the most important aspects of an adsorption material. There are several potential
effects of pretreating char media. One is that pretreatment of media with reducing
agents may remove some acidic functional groups from the surface of the materials,
which would increase the basicity of the media and, thus, increase the pHPZC values.
5
This change in pHPZC should increase the attraction between the media and fluoride ions
without the requirement of metal amendments. Minimal work has been done in this
area.
One study was conducted where activated carbon was treated with iron
ammonium sulfate and sodium dithionite to enhance removal of hexavalent chromium
from water. While the study by Han et al. (2000) did not show success with increased
hexavalent chromium removal, their preliminary work laid the foundation for the
current research seeking to enhance fluoride removal. A second potential effect is that
pretreating the materials with oxidizing agents may add acidic functional groups to the
surface. This should enhance the loading of metals onto the chars and potentially
increase fluoride removal. A few researchers have evaluated oxidation pretreatment
with various carbon-based materials. One study was done to assess whether treatment
with oxidizing agents could alter the lead adsorption capacity of coconut-based
activated carbon (Song et al., 2010). The results demonstrated that lead adsorption onto
the treated activated carbon increased significantly while the surface area changed
minimally and the pHPZC decreased. It was shown that oxidation using boiling nitric
acid increased the acidic oxygen-based functional groups, such as carboxyl and phenol,
on the surface of activated carbon in work done by Liu et al. (2007) and Polovina et al.
(1997). This increase in acidic functional groups is likely the cause of the decrease in
pHPZC resulting from oxidation pretreatment. One impetus for the idea that pretreatment
with oxidizing agents will increase metal loading comes from a study where a lignitebased activated carbon was modified with iron and then used to remove arsenic
(Hristovski et al., 2009). In this study, two methods for treating the starting material
were compared. One method used an alcohol and iron mixture and the other utilized
6
pretreatment with an oxidizing agent, potassium permanganate, followed by iron
amendment.
The material pretreated with potassium permanganate resulted in both
higher iron loading and a higher arsenic removal capacity (Hristovski et al., 2009).
Goal and Objectives
The goal of this dissertation is to investigate fluoride removal technologies in both
laboratory and field settings in order to contribute new information to the area of
fluoride removal for greater availability of safe drinking water and water point
sustainability. Based on previous work and gaps in research the objectives of this
dissertation are to:
1. Present a review of the global failure in implementing safe drinking water
initiatives that exhibit long-term viability and develop ideas for increasing the
sustainability of these initiatives (Chapter 2).
2. Assess the possibility of wood char as a fluoride adsorption substitute for bone
char (Chapters 3 and 5).
3. Investigate ways to improve the fluoride removal capacity and implementation of
bone char (Chapters 3 and 4).
4. Conduct continuous flow field tests on amended and unamended bone and wood
chars (Chapter 4).
5. Evaluate methods for amending wood char with aluminum and iron oxides and
assess the subsequent fluoride removal capacities (Chapters 3, 4, and 5).
6. Test the hypothesis that pretreating wood char with oxidizing agents will decrease
the pHPZC and increase metal loading onto the char (Chapter 5).
7
Overview of Chapters
Chapter 2 reviews literature on the lack of long-term sustainability in water
treatment or supply systems implemented in the developing world. This chapter
discusses past successes and failures and recommends resources to help researchers and
practitioners within the water community achieve sustainable solutions. The chapter
concludes with the suggestion that development and implementation of water treatment
and supply points should not be done only by engineers or, for that matter, any
individual field of study, but instead should be a collaborative effort among diverse
groups such as engineers, business people and social scientists. This collaboration
allows a multitude of ideas, methods and areas of expertise to synergistically develop
solutions that are more sustainable over time.
Chapter 3 presents an investigation of wood char, specifically Eucalyptus robusta,
as a possible substitute for bone char as an adsorption material for areas not amenable to
using bone char. Eucalyptus wood was charred at temperatures ranging from 300 – 600
°C and fluoride removal capacity was tested to find the most effective charring
temperature. While the results showed that 600 °C was the most effective charring
temperature for fluoride removal, all of the wood chars were minimally effective
despite the high specific surface area and pHPZC values, approximately 320 m2/g and
9.6, respectively, at 600 °C. Several experiments were conducted to assess the potential
to improve the fluoride removal capacity of the wood char. In one test wood char was
amended with aluminum nitrate and in another it was pretreated with reducing agents.
The early-stage work with reducing agents did not show promising results and thus, this
8
was not further investigated. On the other hand, the aluminum nitrate amendment
resulted in over 400 % improvement from the low starting fluoride removal capacity of
the wood char alone, although still not reaching the removal capacity of bone char.
Chapter 4 delved further into aluminum amendment of wood char and also added
the study of iron oxide amendment in pursuit of wood chars with higher fluoride
removal capacities. Both aluminum and iron oxide amendment of wood char were
found to increase the fluoride removal capacities while decreasing the pHPZC. One of the
iron oxide amendments provided the largest increase in fluoride removal; however, it
also showed the largest decrease in specific surface area after amendment and therefore,
was the least efficient when fluoride removal was normalized by metal loading. This
research also assessed pretreatment of wood char with oxidizing agents and found that
in several cases, pretreatment increased metal loading onto wood char and, for some
pretreated and amended chars, fluoride removal capacities also increased.
Chapter 5 evaluated fluoride uptake by several materials, including two discussed
in Chapter 3, one produced by an alternative aluminum impregnation method, and one
that is prevalently used for fluoride removal. Based on isotherm batch tests, these
materials were selected for field study in the Rift Valley of Ethiopia. This field work
was done to determine and better understand any differences between material
performance in a realistic field setting versus a laboratory setting. These studies utilized
both batch and continuous flow column studies and assessed whether natural organic
matter in groundwater would compete with fluoride for removal on adsorption
materials. Competing ions and groundwater pH were shown to impact fluoride removal
capacities. In addition, the Rapid Scale Small Column Test approach was shown to
9
apply for fluoride uptake with bone char.
Chapter 6 reviews the key findings of this dissertation relative to technologies for
fluoride removal from drinking water and implementation of safe water initiatives in a
sustainable way.
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14
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15
CHAPTER 2
In Pursuit of Sustainable Water Solutions in Emerging Regions1
Abstract
While lack of access to consistent safe drinking water is estimated to affect nearly 2
billion people worldwide, many of the efforts to solve this crisis have proven to be
unsustainable. This paper discusses some of the reasons for these challenges and suggests
interdisciplinary practices that could be integrated from the very beginning of a water
intervention to achieve long-term success. Of key importance for sustainable water
implementation is an enabling environment that incorporates aspects such as funding, potential
for market development, and supportive governance. While this enabling environment is
acknowledged, the focus of this work is on the integration of three key areas: (i) social and
cultural assessment of behavior and preferences; (ii) market-based implementation approaches
that draw on this knowledge; and (iii) technology development for these markets.
Introduction
The World Health Organization (WHO) and United Nations International
Children’s Emergency Fund (UNICEF) estimate that 780 million people lack access to
improved water sources (WHO and UNICEF, 2012). WHO and UNICEF (2012) also
acknowledge that not all water from improved sources is safe for consumption; an
estimated 1.2 billion people consuming water from improved sources are still exposed
1
Brunson, L.R., Busenitz, L., Sabatini, D.A. and Spicer, P.G. (2013) “In Pursuit of Sustainable Water
Solutions in Emerging Regions.” J. Water, Sanitation Hygiene for Development. 3(4), 489 – 499.
16
to potential health risks from water (Onda et al., 2012). Pursuit of safe drinking water is
further complicated by the fact that 1.4 billion people are living on less than US$1.25
per day, thus increasing the challenge of getting safe drinking water to many
communities (United Nations, 2009).
The focus of this paper is on the development and implementation of sustainable
drinking water solutions for emerging regions. While there are some important overlaps
with sanitation, we focus here on safe drinking water since the proposed market
solutions we explore are, to some extent, unique to this issue. We acknowledge that
improved sanitation plays an important role in access to safe drinking water and that
some of the ideas and tools presented in this work are equally applicable to sanitation
but because of boundary and space constraints, we only address safe drinking water
issues here. We encourage others to extend the ideas presented here to the sanitation
sector.
Implementation Challenges
In response to the lack of safe drinking water, numerous governmental,
university and humanitarian organizations have attempted to implement solutions
ranging from point-of-use (POU) household treatment systems to piped communityscale projects. Despite these good intentions, sustainable water solutions are often not
realized and many challenges still remain. Furthermore, global data on long-term
successes and failures are frequently unavailable because of the expense and challenge
of achieving high-quality, long-term monitoring (Onda et al., 2012;
17
WHO and UNICEF, 2012). Nevertheless, available data and observations indicate that
the water issue is far from solved, in part because of the premature failure of water
schemes (Foster, 2012; Hunter et al., 2010). For example, over the past 20 years an
estimated US$1.2 billion have been ‘lost’ because of wells that have failed prematurely
in sub-Saharan Africa (Schouten and de Jong, 2009).
There are many reasons why water systems fail in developing countries. One
reason is that solutions may not be culturally appropriate or are not selected based on
local preferences and practices, which makes it unlikely that the community will readily
adopt and/or sustain them (Hokanson et al., 2007; Skinner, 2009). For example, if a
society believes drinking warm water causes illness then setting up a system that,
intentionally or unintentionally, results in warm drinking water may produce water that
is safe but is deemed unacceptable by the community (Wellin, 1955; Rogers, 2003).
Some problems are technology related such as when hardware fails or a filter clogs
(Hankin, 2001; Mackintosh and Colvin, 2003; Burr et al., 2012).
Another
common
cause of failure is missing or ineffective supply chains for breakable parts, necessary
chemicals, or replacement filtration media, or when there is a lack of human capital for
maintenance and repairs (Diergaardt and Lemmer, 1995; Oyo, 2006; Clasen, 2009;
Hirn, 2012). Other frequently mentioned causes of failure include lack of education on
the importance of the safe water scheme (Hokanson et al., 2007), minimal capacity and
ownership of the intervention in the community (Breslin, 2003; Skinner, 2009),
aesthetic
issues
such
as
taste
or
smell
of
the
treated
water
(Diergaardt and Lemmer, 1995; Hoque et al., 2004; Wood et al., 2012), disruption of
daily routine (e.g. time or inconvenience) (Hoque et al., 2004; Gupta et al., 2008;
18
Sobsey et al., 2008), costs to consumers (Carter et al., 1999; Gupta et al., 2008;
Sobsey et al., 2008; Wood et al., 2012), and lack of accountability or follow through
between
stakeholders
such
as
government,
community
and
NGOs
(Carter et al., 1999; Harvey and Reed, 2006; Hunter et al., 2010). Table 2.1 summarizes
examples of water projects, primarily failures but also a few successes, along with
reasons for success or failure and suggestions for improvement.
Table 2.1: Examples of water projects in developing countries and possible
improvements that could increase sustainability. Text in grey indicates projects with a
high level of success.
Projectsa
Reasons for failure
or successb
Possible improvementsc
Wells in Africa
(nearly 200,000
wells)
 Lack of funding for
repairs/maintenance and
overall lack of physical
repairs and maintenance
(Reasons implied but not
specifically stated in
source documents; overall
statistic comes from many
different projects and
various reasons for failure
likely exist)
 Market-based solutions to
encourage supply chains and
repair networks
 Social and cultural assessment
research to ensure community
acceptance and buy-in
 (Breslin, 2010)
 (Schouten and
de Jong, 2009)
Water boiling
behavior change
in Peru
 Community members
associated warm boiled
water with sick people and
were therefore, in many
cases, unwilling to boil
water for drinking, despite
the health education
provided
 Intervention was
conducted with the help of
trusted local nurses
 Social marketing was
utilized for chlorine
 Culturally appropriate
education messages
 Initial interventions were
conducted by local NGOs
and then later followed up
 Attention to cultural
assessment and local
information to inform
treatment choice
 (Wellin, 1955)
 (Rogers, 2003)
 Follow up education from
nurses or local project staff
 Address concerns that the
health clinic visitors may not
be representative of the total
local community and engage
in education initiatives outside
the health clinic
 Parker et al., 2006
Household water
chlorination in
Kenya
19
Sources
Projectsa
Possible improvementsc
Reasons for failure or
successb
Sources
Chlorination
project in
Namibia
 Lack of consistent supply
chain for chlorine
 Taste of water after
chlorination was not
palatable for users
 Assess user preferences
through social science
assessment prior to
technology implementation
 Test a business model for
sustaining a consistent supply
of chlorine for water treatment
 (Diergaardt and
Lemmer, 1995)
Bone char use to
remove fluoride
from drinking
water in Ethiopia
 Community unwilling to
use bone char for cultural
reasons
 Lack of education for
communities on system
and effectiveness
 (Abaire et al.,
2009)
Public water
points were
partially broken
or completely
broken at rates of
44% and 24%,
respectively, in
Sierra Leone
 Lack of supply chain for
replacement parts
 Lack of trained persons to
do repairs
 Lack of active
management of water
point
 Social marketing campaign to
raise awareness and encourage
use and purchase of treatment
technology
 Quality control and additional
research on producing highquality bone char for water
filtration
 Ensure active management of
water point
 Set up market-based models
for supply chain and repair
mechanics
 Increase local ownership of
wells
Water filtration
and safe storage
for HIV/AIDS
population in
Zambia
 Implementation in a
population already highly
aware of health issues
 Provision of safe water
storage containers to
provide a storage option
for filtered water
 Time required for use of
water treatment product
 Cost (a possible but less
supported cause)
 Inclusion of a broader
population beyond those that
were referred by health clinics
 (Peletz et al.,
2012)
 More technology development
to reduce time required for
treatment
 Reduce costs
 Marketing strategy based on
more than health outcomes
 (Luby et al.,
2008)
 (Jones,
Christensen and
Thomas, 2008)
 Broken ceramic filter
components
 Filters were considered
slow
 Concern that filter life had
expired
 Technology development to
reduce breakage rates
 Increased supply chain access
and knowledge of where the
supply chain exists
 Increased education in user
households
 Amend expectations for life of
the filters
 (Brown et al.,
2009)
Intervention to
improve safe
water access
through sale of
PUR sachets in
Guatemala
Ceramic water
filters for pointof-use water
treatment in
Cambodia
aMost
 (Hirn, 2012)
examples listed here are not of complete failure but partial (e.g. 200,000 wells failed in Africa though some are
still functioning and 69% of ceramic water filters are no longer operational while 31% are still in use). Shading
indicates water interventions that were primarily successful.
bReasons for failure are stated or implied by the source document.
cPossible improvements are in part suggested in the original source documents and in part suggested by the authors of
this work based on insights gleaned from reviews of failed and successful projects.
20
Sustainable Development
The failure to achieve sustainable safe water solutions is well known, but much
remains unknown about how to move forward. Breslin (2010) argues for increased
attention to monitoring, including a focus on realistic success metrics over the long
term, transparency and accountability, and assessing where funding comes from and
how it is leveraged. Hokanson et al. (2007) studied several specific water purification
technologies and several geographic regions and made suggestions including:
appropriate technologies should be used, community training should be increased, and
more efforts should be made to improve community buy-in. Building on these
important efforts, we argue that formulation and implementation of sustainable water
solutions requires the blending of social science (to understand the human dimensions),
business and economic capacity (to aid in market-based implementation and assuring a
consistent supply chain), and science and engineering (to aid in technology
development). These three components of the system are often discussed individually
but rarely integrated in the literature. Indeed, social and cultural assessments are often
quite hostile to market-based solutions (e.g. Goldman, 2006). While a number of water
system approaches provide safe water, the ideas presented in this work apply primarily
to POU and community-scale safe water systems. Figure 2.1 shows how the
overlapping disciplines might look when working together within a given enabling
environment, as discussed below. This collaborative effort is proposed not as a one-time
occurrence in initial product development, but instead as something that should occur
throughout the duration of each initiative.
21
Figure 2.1: Diagram showing the three discussed areas crucial to successful
implementation, their overlap where a sustainable outcome should be obtained, and
their embedding in a broader systems perspective (the enabling environment).
Mihelcic and Trotz (2010) argued that the “Application of sustainability to
engineering projects thus requires more emphasis on integrating and balancing human
and societal considerations with technological and economic consideration.” We build
on this position and assert that the integration of disciplines should be conceptualized
not as three separate sub-projects working in one area, but instead as three distinct areas
of thought and expertise working collaboratively on one project with a common goal of
building a sustainable solution. Table 2.2 suggests tasks that fall into each aspect of this
integrated approach. We propose an integrative vision of these solutions by embedding
them in the enabling environment, which incorporates aspects such as politics,
economics, available resources, legal frameworks, and funding opportunities. In order
22
for projects to be successful an enabling environment must exist that is conducive to
sustainable safe water supply solutions. This cannot, of course, be presumed, but in this
paper we assume that such an enabling environment exists and we focus on the other
key elements to achieving sustainable water solutions. We focus here on local context,
but we expect that this will lead to a more comprehensive approach that will consider
these local systems in a global context that includes donor communities and
governments (Breslin, 2010; Hunter et al., 2010).
Table 2.2: Tasks that fall into each component of synergistic research towards
developing sustainable research and water access or treatment implementations.
Social and cultural
assessment
Market-based
implementation
Technology
development
 Assessment of local
practices regarding
drinking water
 Assessment of
willingness to pay
 Assessment of drinking
water aesthetic
preferences
 Values inventory
 Determination of health
needs, goals, beliefs, and
behavior
 Business models
research
 Market research
 Economic assessment
 Implementation
through social
entrepreneurship
 Cost assessment using
life cycle costs
approach
 Supply chain analysis
and support
 Lab testing
 Field technology
assessment
 Improved or new
technology
development
 Life-cycle assessment
 Assessment of locally
available materials and
processing capabilities
Social and Cultural Assessment
Critiques of development initiatives abound within the social sciences (e.g.
Ferguson, 1994; Scott, 1999; Goldman, 2006) especially criticisms of the failure to
consider local social and cultural realities. Unfortunately, these critiques have seldom
been applied in improving development initiatives such as access to safe water.
23
Understanding the social and cultural context in which people may care about drinking
water quality is an obvious, but often neglected, dimension of any sustainable safe
water scheme. In a development context, technological and market-based approaches
presume certain forms of social and cultural orientation, which may not always be
warranted. For example, Mary Douglas, a social anthropologist, points out that
sometimes apathy of communities, often caused by helplessness and an inability to
envisage a way out of the current life situation, can be a major deterrent to
implementation and success of development projects (Douglas, 2004). When properly
designed and implemented, social and cultural assessment should help us better
understand the human system in which an initiative is attempting to intervene.
Social and cultural assessment is commonly conducted by anthropologists,
sociologists, or psychologists (and at times engineers) using methods such as
observations, surveys, interviews and focus groups. Huber et al. (2011) discuss a
method that can be used to conduct this research and McConville and Mihelcic (2007)
provide a sample list of information to gather. These tools can reveal where people get
affirmation or criticism of their behaviors, what beliefs cause people to accept or reject
ideas, and information about attitudes, preferences, aspirations, and other factors that
influence current and potential behavior. Formative research is needed to assess needs
as well as community wishes. This process can be daunting and several helpful sources
of information are available that offer guidelines and approaches for this. The
Participatory Hygiene and Sanitation Transformation (PHAST) approach is one method
for working with a community to plan a project and a helpful guide is available from
WHO titled PHAST step-by-step guide: A participatory approach for the control of
24
diarrheal disease. Safe Water Systems for the Developing World: A Handbook for
Implementing Household-Based Water Treatment and Safe Storage Projects from the
Centers for Disease Control and Prevention offers information about how to conduct a
focus group and includes sample questions. Knowledge, Attitudes and Practice (KAP)
studies are another method for gathering data: Two helpful resources, though not
specific to water, are ‘The KAP Survey Model’ from Médecins du Monde and ‘A Guide
to Developing Knowledge, Attitude and Practice Surveys’ from WHO.
A growing number of water researchers have gravitated toward cognitive and
behavioral theories of behavioral change, often derived from health psychology, to
understand problems in international water development (e.g. Heierli, 2008). Others
have found the inductive community-based approaches of anthropology to be central to
their efforts (e.g. Mihelcic et al., 2007; Ramaswami, et al., 2007). Thus, social and
cultural assessment plays a key role in safe water development because the research in
this area assesses community knowledge, beliefs, behaviors and desires and then
influences the education, information sharing, and motivational tools for health
behavior change that have been argued to be a critical component of water treatment
projects as a whole (Hokanson et al., 2007). Health behavior change is a difficult topic
at times because it can seem paternalistic or offensive. However, it is possible to
motivate, as opposed to force, positive health-related behavior modification with a
minimum level of community change or invasiveness if researchers and practitioners
gain a solid understanding of the community and its history and beliefs, and follow
ethical development guidelines (e.g. Gasper, 2004, 2012; the International Development
Ethics Association “http://secure.pdcnet.org/idea/About-Us).
25
Social and cultural assessment, if well done, should also inform both market
analysis and technology selection and development. If we take seriously the idea that a
community or family should be able to have a voice in selecting their water treatment
technology or water source, social sciences are essential to cultivate this participation
(McConville and Mihelcic, 2007; Skinner, 2009). Heierli suggests one of the most
important things to do in determining implementation methods for a community is to
gain a thorough understanding of the culture and values of the community in which
practitioners and researchers are working (Heierli, 2008). Rogers (2003) suggests new
technologies are better disseminated if the change in behavior required is minimal when
compared with previous behavior; thus, understanding patterns of behavior, behavior
stimuli, and habits of the community members is key. In direct relation to water, we
know that household water treatment technologies are more likely to be accepted and
used if they are easy to use and require minimal time (Sobsey et al., 2008). Thus, social
and cultural information gathering and assessment is a critical aspect of an integrated
water implementation. The knowledge gained about local communities is crucial to
understanding the local markets that should inform the selection or design of
technologies.
Market-based Implementation
There are growing signals that market-based solutions may be an important
avenue through which sustainable drinking water issues can be addressed. For example,
Prahalad (2009) noted that there are millions of people living in poverty that are
becoming viable consumers of scalable consumer items sold through sales techniques
26
and appropriate marketing. For the world’s 4 billion poorest people, the water market is
approximately $20 billion (Hammond et al., 2007). Thus, in the larger ecosystem,
utilizing market approaches can be beneficial for poverty reduction and global
economic growth along with increased sustainability of access to safe water.
There are several areas where an entrepreneurship perspective contributes to water
research and implementation. The process of analyzing how the various pieces of a
business model could work or how they need to be adjusted to better facilitate the
customer value proposition, the inter-workings of the various exchange partners and
how
money
flows,
can
be
highly
constructive
(McGrath,
2010).
Rural Water Supply Network (2010) and Foster (2012) suggest that exploration and
testing of market-based solutions for rural safe water in developing countries is helpful
for addressing water sustainability concerns. Markets readily benefit people by offering
competition, which drives down prices and encourages continued innovation based on
customer demand (Wolff, 2012). Furthermore, market-based systems provide strong
encouragement to maintain, repair and upgrade existing systems as individual
livelihoods depend on ongoing operations. The use of market-based models for water
implementation also has the potential to solve issues such as missing supply chains,
willingness to use technologies, and marketing concerns (Foster, 2012). Skinner (2009)
suggests it is helpful to have a viable market that ensures availability of a repair
person(s) and replacement supplies to encourage fair pricing and options for consumers.
And Oyo (2006) indicates that, while private operation of supply chains for well and
pump spare parts in Africa is not currently the norm nor always the best option, it could
be the ideal option for sustainability if certain operating criteria are met.
27
While market-based implementation is certainly gaining traction, constraints
include political instability, corruption, and lack of literacy (Hokanson et al., 2007).
Government or NGO projects often run out of funding because of political or economic
shifts, thereby jeopardizing the success of the project (Banerjee et al., 2007). In contrast,
market-based solutions that are financially self-sustaining have the potential to be much
more enduring without concerns about donor or government funding. In market-based
systems, including POU or community scale, financial concerns must be addressed to
remain operational. The means of economic exchange and payment systems between
suppliers and users must also be accommodating and fluid. Therefore, aside from
studying business model alternatives, business experts on water research teams need to
understand what can add value for customers, how it can be delivered in a way that
meets the needs and wants of customers, and cost/payment mechanisms.
Fonseca et al., (2011) and Burr et al., (2012) offer an in-depth discussion of the life
cycle costs approach and suggest that it takes into consideration water point/treatment
costs including minor operations and maintenance and major capital maintenance,
capital investment and both direct and indirect support. Fonseca et al. (2011) provide
information on accounting strategies and guidance on how to conduct a life cycle cost
assessment.
There is increasing movement towards charging water consumers enough such
that full cost recovery becomes a viable option, but in many cases users do not have the
ability to pay at this level (Banerjee and Morella, 2011; Foster, 2012). Thus, subsidies
may
be
needed
to
complement
local
systems
in
certain
cases
(Banerjee et al., 2007; Kremer et al., 2010; Foster, 2012). Due to the diversity of
28
funding sources and the complexities of implementation, options such as hybrid
organizations, subsidies from governments or NGOs, paying in installments, and other
creative solutions could be explored for the implementation of some market-based
solutions (Gupta et al., 2008). Further discussion of issues with willingness to pay,
costing and potential options for private water business can be found in Foster (2012)
and Kols (2011) and there is an ongoing need for research on private sector water
options (Harvey and Reed, 2006).
Market-based solutions can improve sustainability of water supply as well as
benefit communities by employing local people and expanding markets. However, it is
important to acknowledge that market-based approaches are not ideal in every situation.
For example, willingness to pay for water treatment in some areas can be quite low, and
it was reported by Null et al. (2012) that the short-term uptake of water treatment
options was much higher if items were given away free or were very inexpensive, thus
not supporting a market-based approach. However, Baumann (2006) suggests that the
market for boreholes in Africa is massive and increasingly the private sector, in
particular indigenous drilling companies, are best suited for meeting this need.
Additional research on payment options is needed to inform market-based solutions and
the ‘best option’ no doubt depends on local culture, habits, preferences, markets, etc.
Related to these market-based approaches is social marketing. Social marketing
is the application of marketing principles to influence social behaviors and accomplish a
social good (Andreasen, 1994). Furthermore, social marketing seeks to benefit the target
market and even society as a whole, not the marketer as done in commercial marketing.
Kotler & Zaltman (1971) were among the first to characterize social marketing as ‘A
29
bridging mechanism which links the behavior scientist’s knowledge of human behavior
with the socially useful implementation of what that knowledge allows’. With public
health issues, social marketing has been employed to help implement programs aimed at
broad-based behavioral change (Andreasen, 1994; Rogers, 2003).
Social marketing is a tool that allows us to consider ways of combining social
and cultural assessment with market-based implementation and dissemination of
treatment technologies or safe water sources. Social marketing can be powerful when
used by the private sector or by NGOs, such as Population Services International, for
marketing safe water solutions based on careful social and cultural assessment, though
it should not necessarily be considered a complete substitution for other behavior
change activities (Banerjee et al., 2007; Foster, 2012; Wood et al., 2012). Sometimes
combining more than one health-related initiative together can be beneficial. For
example, Peletz et al. (2012) found that implementing household water treatment filters
with HIV-positive mothers who had young children resulted in an 82% use rate and
improved drinking water quality, confirmed by bacteriological improvements. The
authors suggest this may have resulted from an increased awareness of health-based
concerns in HIV-positive populations, thus, leading to an increased willingness to use
health improvement interventions (Peletz et al., 2012).
Good market-based approaches have the best chance of success when they are
built on a solid assessment of the culture. Furthermore, to have water treatment products
that are effective, efficient and culturally acceptable is imperative for developing
market-based systems. At the same time, social and cultural assessment and market
analysis are useless without sustainable technology, to which we now turn.
30
Technology Development
The ability to supply or treat water is essential to addressing the safe water
availability issue, which is the focus of this paper. But if we are to take seriously the
critiques of water development that formed the motivation for this essay, it is clear that
technology development is just one piece of the interdisciplinary effort. There are two
key aspects to a discussion of technology in the framework of building sustainable
water solutions.
First, technology research, development, and assessment can be more effective
and powerful when guided by information learned through social and cultural
assessment (i.e. local cultures and taboos) and when, in consideration of the local
economy, technology’s place in local markets is taken into consideration. As mentioned
in Table 2.1, sometimes projects fail because a technology is not appropriate for the
local community. For example, research using bone char to remove harmful fluoride in
a community that has a cultural preference against animal bones is not appropriate, even
if the approach is technically viable (Abaire et al., 2009). Assessment of community
preferences should guide technology selection/development with the assumption that a
community or family may be responsible for funding operations and maintenance as
well as physically collecting/treating water. Selection of locally produced versus
imported technology is often a discussion of cost and availability of materials, but it is
imperative to also consider community members’ preferences as they will influence
what technologies will actually be utilized. Additionally, depending on what the local
31
market situation is, the ‘best’ answer may be one that does the most to facilitate not
only increased access to safe water but also to boost the local economic market.
A second key aspect of research and selection of technologies is that in some
cases the necessary technologies either do not exist or would benefit from continued
research for improvements. The literature presents some cases where projects fail as a
result of technical issues or where technologies could be improved to make them more
user-friendly, sturdy, effective, etc. For example, Hokanson et al. (2007) point out that
UV disinfection is highly effective for pathogen removal, but that there are several
drawbacks including: the necessity of a power source, the requirements for quality of
the source water, and the fact that no visible change occurs during the treatment
process. Gupta et al. (2008) suggest the need for additional engineering to make chulli
water filters more durable and reliable, and Baumann (2006) suggests that decreasing
the cost and increasing the efficiency of borehole production, specifically in Africa,
would improve access to safe water in Africa.
Sustainability Tools
A number of tools can guide the kind of integrations we propose here. The work
by McConville and Mihelcic (2007), which develops a framework for looking at factors
that affect sustainable development of water and sanitation projects, provides practical
steps to be utilized – specifically Table 1 which suggests helpful tasks within five life
stages of a project. The Triple-S (Sustainable Services at Scale) initiative website, led
by the International Water and Sanitation Center, contains tools, ideas, case studies, and
training options and is available at http://www.waterservicesthatlast.org. Finally,
32
Skinner (2009) discusses ways to make Africa’s waterpoints more sustainable including
focusing on quality control of implemented technology, waterpoint management and
ownership, and use of local markets to support maintenance and repairs. This work
offers a table containing 30 elements that are suggested as necessary for success for
sustainable safe water in African communities.
Practical Implications
There are many lessons to be learned from the individual disciplines discussed
above, but more important than the individual disciplines learning from one another in
isolation is the synergistic impact of collectively designing, evaluating and
implementing sustainable solutions for access to safe water. We have argued for the
importance of including at least the three disciplines (social science, business and
technology) into a safe drinking water initiative from the very inception of the project in
order to ensure that each step of the process is influenced by all areas. We argue that
failure to collectively include multiple areas from the onset will lead to suboptimum
solutions. For example, through this literature review the importance of conducting a
thorough market analysis of the target communities or households has emerged.
This type of analysis could be done in isolation, but to be most effective requires
a solid social and cultural understanding of human behavior, desires and motivations,
the willingness/ability to pay for safe water, and social marketing to better inform the
targeted populations. At the same time, social and cultural assessment could be done by
a
social
scientist
who
does
not
fully
understand
the
business/supply
chain/entrepreneurial elements and thus would fail to incorporate critical information
33
necessary to assess these aspects. Finally, this step, even if forged by both of these
disciplines, may lack knowledge of what technologies might be available or feasible.
We strongly encourage researchers and practitioners to fully adopt this integrated
approach into their program design, implementation and assessment, and have provided
a number of tools that can facilitate these processes in most initiatives, even in the
absence of specialized expertise on a team. We further suggest that funders and
government organizations look for these elements in project proposals or evaluations
and consider funding and supporting activities in part based on evidence showing
careful consideration of these aspects of sustainable development.
Conclusion
Identifying a successful solution that is appropriate for each context is certainly
a challenge. While a multitude of excellent organizations have been implementing
solutions to the water crisis for decades, an alarming number of past projects are not
demonstrating long-term sustainability. We argue that it is imperative that researchers
and practitioners work to find best practices, integrate skills and ideas from many fields,
and put them together to research and create synergistic solutions for safe water needs
in communities around the world. We posit that the combination of social and cultural
assessments, market-based solutions and technology development can provide a
powerful approach for achieving sustainable safe water solutions by allowing each
approach to bring unique tools and methods to the project that inform and influence the
direction and outcome of the project as well as the actions of the other disciplines.
Future research on the methods and outcomes of designing, implementing and assessing
34
this integrated approach will be helpful to the water community as it seeks ways to
further improve sustainability of water supply/treatment implementation.
Acknowledgements
The authors gratefully acknowledge partial funding from the US Environmental
Protection Agency and the US National Science Foundation that contributed to this
work. This publication was developed under STAR Fellowship Assistance Agreement
no. 91731301 awarded by the US Environmental Protection Agency (EPA). This work
has not been formally reviewed by EPA, and the views expressed in this paper are
solely those of the authors and the EPA does not endorse any products or commercial
services mentioned in this publication. The authors also wish to acknowledge partial
funding from the OU WaTER Center and the University of Oklahoma in support of this
work.
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42
CHAPTER 3
The Role of Surface Area and Surface Chemistry during an
Investigation of Eucalyptus Wood Char for Fluoride Adsorption from
Drinking Water2
Abstract
This work studied the use of eucalyptus wood char as an adsorption material for
fluoride removal from drinking water and investigated options to improve the removal
capacity of bone char to increase availability of safe water. Upon increasing charring
temperature, the specific surface area of the eucalyptus wood char increased from 0.9 to
327 m2/g; however, the fluoride removal capacity of the unamended eucalyptus wood
char was negligible for all charring temperatures. Amendment of the wood char with
aluminum oxides increased the fluoride removal capacity to 1.6 mg/g, demonstrating a
significant improvement relative to the unamended wood char. Although this adsorptive
capacity is not yet as high as values reported for bone char, it is higher than the capacity
of other media currently in use in some locations. Thus, the aluminum oxide modified
eucalyptus char may be a helpful alternative for fluoride removal in areas where bone
char is not a viable option. This work also studied the possibility of altering the pHPZC,
initially 9.6 and 8.2 for wood and bone chars, respectively, to enhance the electrostatic
2
Brunson, L.R., and Sabatini, D.A. (2014). “The Role of Surface Area and Surface Chemistry during an
Investigation of Eucalyptus Wood Char for Fluoride Adsorption from Drinking Water.” J. Environ. Enr.
Accepted.
43
attraction between the media and fluoride in water. The use of reducing agents actually
lowered the pHPZC to less than 8 for both types of char but increased the fluoride
adsorption capacity of the bone char by approximately 25% while causing minimal
change to the wood char capacity.
Background
Naturally occurring fluoride is a major drinking water quality concern
contributing to the global water crisis; worldwide an estimated 200 million people are
affected, including many living in the Rift Valley of Africa (Coetzee et al., 2004;
Nigussie et al., 2007; Fawell et al., 2006) as well as people living in India, China, and
several areas of the United States (Amini et al., 2008; McGill, 1994). When fluoride is
consumed at levels above the World Health Organization (WHO) standard of 1.5 mg/L,
dental and skeletal fluorosis can occur (World Health Organization, 2011). Beyond
discoloration of teeth, fluorosis can cause tooth sensitivity and pain, stiffness and
defomities in bones and joints (Fawell et al., 2006; Fewtrell et al., 2006; Malde et al.,
2011). Not only is this a significant health issue, but social and financial impacts can
result from the social stigma of dental fluorosis as well as from physical disabilities that
impact subsistence farmers and wage earners (McGill, 1994; Tonguc et al., 2011).
Water treatment methods investigated for fluoride removal include: ion
exchange, electrolytic defluoridation, filtration, and precipitation (Abe et al., 2004;
Fawell et al., 2006; Ayoob et al., 2008; Gwala et al., 2011). Adsorptive filters are one of
the most commonly used methods, due in large part to the ability to operate filters
44
without any stirring or mechanical requirements once water is pumped from the ground
(Daifullah et al., 2007).
Commonly used fluoride adsorptive materials include activated alumina, bone
char, and clays (Fawell et al., 2006; Ayoob et al., 2008; Mlilo et al., 2010). Activated
alumina is a high surface area removal material that functions best at low pH values and
is suggested to have a field removal effectiveness of 1 mg of fluoride per gram of
material (Fawell et al., 2006; Ayoob et al., 2008). Activated alumina is not commonly
produced in many developing countries and would therefore need to be imported, which
would likely increase the cost of water filtration. Clays are locally available in many
locations, but their chemical compositions are highly variable and removal capacities
are typically low, much less than 1 mg/g (Coetzee et al., 2004; Fawell et al., 2006).
Bone char, widely investigated for adsorptive removal of fluoride, can be produced in
developing regions from waste animal bones, is relatively inexpensive, and is composed
primarily of hydroxyapatite, Ca10(PO4)6(OH)2, (70-76%), carbon (8-11%), and calcium
carbonate (7-9%) (Larsen et al., 1994; Wilson et al., 2003; Kaseva, 2006; MedellinCastillo et al., 2007). The fluoride removal capacity of bone char has been attributed to
several mechanisms including: the exchange of fluoride for hydroxyl ions due to their
similar charge and hydrated radii (3.52 and 3.00 Å, respectively) (Nightingale, 1959;
Fan et al., 2003; Kaseva, 2006) and exchange of carbonate for fluoride (Bhargava and
Killedar, 1991).
Charring animal bones is helpful because it produces a large specific surface
area. Medellin-Castillo et al. (2007) found the bone char surface area to be 104 m2/g
which is comparable to the 150 m2/g found by Chen et al. (2008) and the 98 – 112 m2/g
45
shown by Brunson and Sabatini (2009). Variations in the surface area are found based
on bone type, charring temperature and preparation method. Another measurable
characteristic of bone char is its point of zero charge (pHPZC) which Medellin-Castillo et
al. (2007) found to be 8.4, consistent with the 8.2 pHPZC found by Brunson and Sabatini
(2009). In solutions with pH values below the pHPZC the surface of a material will be
positively charged, further increasing in charge with any additional lowering of solution
pH. In contrast, for pH values above the pHPZC, the surface of a material will have a net
negative charge. Therefore, a material with a high pHPZC is expected to work best for
removing negatively charged fluoride ions from water because it will have a strong
positive surface charge at typical groundwater pH values (ranging from 6.0 to 8.5).
In some communities, for religious or cultural reasons, people are not amenable
to using bone char for water treatment (Fawell et al., 2006). A potential alternative is to
use a carbon-based material, such as wood char. Wood is primarily composed of lignin
(18-35%) and carbohydrates (65-75%) (Pettersen, 1984). However, chemical
composition and other characteristics of wood and wood chars vary based on location,
soil type, season, age and type of wood (Pettersen, 1984). James et al. (2005) showed
that the specific surface areas of several types of wood naturally charred in forest fires
ranged from 1.7 to 85 m2/g, while woods charred under laboratory conditions at varying
temperatures had specific surface areas in the range of 1.0 to 397 m2/g. This suggests
that surface area is highly variable based on wood type, location, and charring
temperature and method. Previous studies on spruce wood (Tchomgui-Kamga et al.,
2010) and unspecified wood types (Abe et al., 2004) suggested that unmodified wood
char has merit for study, but may not be as effective at removing fluoride as bone char.
46
Abe et al. (2004) found in a study of twelve char materials, that the three different wood
chars exhibited higher fluoride removal than charcoals or petroleum coke but much less
than that of bone char (e.g. 13% removal for a wood char versus 82% removal for bone
char).
Several approaches can be tested for improving the fluoride removal capacity of
wood char. Based on the success of aluminum-based materials, one potential way to
improve fluoride removal of wood char is to amend it with aluminum oxides. LevyaRamos et al. (1999) impregnated coconut shell-based activated carbon with aluminum
nitrate and found that the fluoride removal capacity increased 3 to 5 times after
aluminum amendment. Tchomgui-Kamga et al. (2010) tested the potential to remove
fluoride by impregnating spruce wood with aluminum chloride. Their work suggested
that the best results, approximately 3.8 times better than charred spruce alone, were
obtained when wood was impregnated with both aluminum and iron. Based on these
two studies, further study of aluminum amendment on wood char for fluoride removal
is helpful.
Another option for enhancing the fluoride removal capacity of wood char may
be to alter the pHPZC of the char. The suggestion is that if the pHPZC is raised, then the
stronger positive charge on the sorbent surface at typical groundwater pH values will
improve fluoride removal. Treating activated carbon type materials with reducing
agents may remove acidic functional groups from the surface, which would therefore,
increase the basicity and raise the pHPZC. Han et al. (2000) treated granular activated
carbon (GAC) with two reducing agents, iron ammonium sulfate and sodium dithionite,
with the intention to enhance removal of hexavalent chromium by altering the surface
47
chemistry of the GAC. In this case, removal of chromium was not improved after
treatment with reducing agents; the treatment effects on characteristics such as pHPZC
and specific surface area were not reported. Thus, to date, research has not fully
evaluated the concept of using reducing agents to alter the pHPZC of charred media,
making this a fruitful area of study.
Based on previous research, two key items that make media effective at fluoride
removal are high specific surface area (often resulting from materials with high internal
porosity) into which fluoride can be sorbed and a surface chemistry that attracts
fluoride. Thus, the goal for this work is to assess whether wood char can be produced
that has both high specific surface area and desirable surface chemistry that is
conducive to fluoride removal, preferably approaching or exceeding the effectiveness of
bone char. Two key hypotheses were tested to gain traction towards this goal. The first
is that amending wood char with aluminum nitrate will improve the fluoride removal
capacity due to the attraction between negatively charged fluoride ions and the
positively charged metal oxides. The second is that treatment with reducing agents will
remove oxygen from the surface, which will reduce the amount of acidic functional
groups and, therefore, increase the surface basicity of the sorbent and the pHPZC. It is
hoped that raising the pHPZC will increase the fluoride removal capacity due to the
resulting positive surface charge at groundwater pH levels. This work is unique due to
the study of aluminum amendment using aluminum nitrate with eucalyptus wood
charred at a range of charring temperatures and investigation of altering the wood char
pHPZC through the use of reducing agents. Another key factor this work addresses is the
importance of looking for solutions that are locally available and inexpensive for
48
fluoride removal in developing regions. Eucalyptus wood is prevalent throughout
Ethiopia, a country with a high prevalence of fluoride in ground water, where it grows
quickly and can continue growing even in marginal conditions (Dessie and Erkossa,
2011). Therefore, eucalyptus wood (Eucalyptus robusta) is the biomaterial that was
charred in this work to allow for assessment of the effects of charring temperature on
specific surface area and surface chemistry and of the potential for utilizing wood char
or amended wood char as an alternative to bone char for fluoride removal.
Materials and Methods
Materials
Eucalyptus robusta was purchased from Mezozoic Landscapes, Inc. in Lake
Worth, Florida and fish bone meal was purchased from Peaceful Valley Farm & Garden
Supply in Grass Valley, California. Raw media were charred at temperatures ranging
from 300 °C to 600 °C for four hours based on previous charring work by Brunson and
Sabatini (2009). For charring, media were placed in clean and unglazed porcelain
containers with lids and placed in a temperature controlled Thermolyne oven. After
charring, media were cooled to room temperature, crushed with a mortar and pestle, and
sieved to achieve a size range of 180 – 425 µm. Materials were rinsed with deionized
water to remove fines and stored in clear, sealed plastic bags.
Surface Modification
A 175 mg/L aluminum solution (0.0065 molar) was prepared using aluminum
nitrate and the pH was adjusted to 3.5 with 4-Morpholineethanesulfonic acid based on
49
work done by Levya-Ramos et al. (1999). The solution, 417 mL, was combined with 25
grams of charred media in a glass container and shaken for five days. After shaking,
media were filtered, dried at 100 °C, rinsed with deionized water until the pH of the
solution was consistent and then dried again. Treatment of media by using reducing
agents was conducted based on a modified method from Han et al. (2000) where 0.1
mM solutions of sodium dithionite or iron ammonium sulfate were prepared in
deionized water. The solutions were mixed with 25 grams of charred material and
shaken for 24 hours. Media was then filtered from solution, dried, and rinsed with
continuously flowing deionized water until pH readings were consistent.
Batch Isotherms
Batch tests were conducted by placing 0.5 grams of material in jars and adding
fluoridated solutions with concentrations ranging from 0 to 100 mg/L to the jars. Jars
were then placed on a shaker table and shaken at 200 shakes per minute for 24 hours;
preliminary tests showed 24 hours allows equilibrium to be reached. Solutions were
filtered through Whatman 15 cm ashless paper into clean jars and sealed until pH and
fluoride measurements were taken. Batch studies were run in triplicate to allow for
statistical analysis and all solutions used, unless otherwise specified, were adjusted with
NaCl to an ionic strength of 0.05 molar. Batch tests were conducted at room
temperature (approximately 22 °C) and, unless otherwise specified, starting pH values,
which were not controlled or adjusted, ranged from 5.5 to 7.0.
50
Material Characterization
Surface area and pore size distribution measurements were conducted using a
Quantochrome 2000E instrument. Specific surface area was determined using a five
point measurement system at 77 °K based on the theory of Brunauer, Emmett, and
Teller (Brunauer et al., 1938). Pore size distribution was assessed using a twenty point
measurement, including both adsorption and desorption, with P/P0 values ranging from
0.25 to 0.99 and the Density Functional Theory method. Media pHPZC values were
tested using the drift method (Babic, 1999). Media were added at 0.2 grams per bottle to
sets of 12 bottles containing 50 mL solutions that had been pH adjusted to values
ranging from 2.5 to 12. After media were added the bottles were sealed and shaken for
24 hours. After 24 hours the final pH was tested. Final versus initial pH values were
graphed and the plateau was assessed to determine the pHPZC (Babic, 1999). Tests were
conducted in triplicate.
Analysis
Fluoride samples were combined in a one to one ratio with Total Ionic Strength
Adjustment Buffer, composed of cyclohexylenedinitrilotetraacetate, sodium hydroxide,
sodium chloride, and glacial acetic acid and mixed with samples prior to analysis to
ensure consistent ionic strength and pH for accurate readings. Measurements were
conducted using a fluoride specific electrode connected to an Orion pH/ISE meter. A
HACH HQ30d pH and Multi-Parameter Meter was used to measure pH values.
51
Isotherm data were analyzed using the Freundlich equation as shown in Equation 3.1
where Ce is the equilibrium concentration after an isotherm test, Qe is the amount of
chemical removed per mass of media, Kf is related to adsorption capacity and 1/n
suggests the adsorption intensity (Fan et al., 2003; Kamble et al., 2007). SigmaPlot
(Version 12.0) was used to performed regression analysis in order to calculate Kf and
1/n to p = 0.05.
Qe = KfCe1/n
Equation 3.1
The Freundlich isotherm assumes a heterogeneous media surface accounting for the
nonlinear nature of the adsorption. The equilibrium fluoride concentrations studied,
which are of environmental relevance, did not achieve an adsorption plateau and thus
the data were not analyzed by the Langmuir isotherm.
Results and Discussion
Material Charring
This is one of the first research efforts to assess the ability of eucalyptus wood
char to remove fluoride from drinking water. Results of batch isotherms show that
unamended eucalyptus wood, charred between 300 and 600 °C, demonstrates negligible
fluoride adsorption (Table 3.1); the highest Freundlich adsorption value, Kf =0.02, was
obtained at the charring temperature of 600 °C. Table 3.1 also shows the fluoride
removal capacity of bone char compared with unamended wood char. For example, at
Qe,10 (where Ce equals 10 mg/L in the isotherm), values are 0.11 (600 °C wood char)
and 3.9 (500 °C bone char) mg/g. The most likely reason for the significant difference
in fluoride removal capacity between wood and bone chars is the favorable chemical
52
composition of the bone char, primarily hydroxyapatite, versus wood char, which is
primarily carbon (Benaddi et al., 2000; Wilson et al., 2003; Medellin-Castillo et al.,
2007). This interpretation is reinforced by the fact that the wood char, while exhibiting
much less fluoride adsorption, actually has a higher surface area than the bone char as
discussed in a later section. The minimal fluoride removal capacity of unamended wood
char, even for a range of charring temperatures, is consistent with the work of LevyaRamos (1999) who found the maximum fluoride adsorption capacity of activated
carbon produced from coconut shells to be 0.5 mg/g. Additonally, Abe et al. (2004)
found that out of six tested activated carbon type materials, the highest fluoride
adsorption value obtained was 0.34 mg/g. Ayoob et al. (2008) offered that fluoride has
a high affinity for metals, which means that the nonmetallic sites on materials such as
carbon-based media have low affinities for fluoride even when these media have very
high surface areas.
Aluminum Amendment
In an effort to improve the surface chemistry of the eucalyptus chars they were
amended with aluminum with the starting material being aluminum nitrate. Table 3.1
and Figure 3.1 show the results of batch isotherms for wood char amended with
aluminum oxides (Al wood char) at a range of charring temperatures. Adding aluminum
increased the fluoride removal capacity by approximately 6 times, from 0.11 to 0.72
mg/g at Ce=10 mg/L.
53
Table 3.1: Freundlich constants calculated from batch tests investigating fluoride
removal of aluminum amended and unamended wood and bone chars.
Freundlich Constants
Media
Kf
(mg/g)/(mg/L)1/n
Not quantifiable
1/n
r2
-
Qe1.5 at Ce
= 1.5 mg/L
-
Qe10 at Ce
= 10 mg/L
-
Not quantifiable
Not quantifiable
Not quantifiable
-
-
-
300 °C
Wood Char*
400 °C
Wood Char
*
500 °C
Wood Char
<0.01 ± 0.03
1.35 ± 0.03
0.99
0.00
0.01
600 °C
Wood Char
0.02 ± 0.01
0.60 ± 0.12
0.92
0.04
0.11
500 °C
Bone Char
1.80 ± 0.20
0.33 ± 0.04
0.97
2.10
3.90
300 °C
Al Wood Char
0.04 ± 0.02
0.58 ± 0.11
0.99
0.05
0.14
400 °C
Al Wood Char
0.04 ± 0.01
0.74 ± 0.08
0.98
0.06
0.24
500 °C
Al Wood Char
0.12 ± 0.04
0.48 ± 0.05
0.99
0.24
0.61
600 °C
Al Wood Char
0.15 ± 0.02
0.69 ± 0.04
0.94
0.20
0.72
500 °C
Al Bone Char
1.40 ± 0.10
0.42 ± 0.02
0.99
1.70
3.08
500 °C
**
<0.01 ± ≤0.01
0.98**
0.97
0.00
0.02
0.02 ± ≤0.01
0.98 **
0.73
0.04
0.24
0.02 ± ≤0.01
0.65 **
0.92
0.03
0.10
0.17 ± ≤0.01
0.65 **
0.99
0.23
0.77
500 °C
600 °C
600 °C
*These
Wood Char
Al Wood Char**
Wood Char
**
Al Wood Char**
chars removed such small quantities of fluoride that the Freundlich constants could not be calculated.
**For this data the 1/n Freundlich values have been forced to a common value so that the K values can be directly
f
compared. Due to process of forcing the 1/n values to a common number there is no error for these 1/n values.
As can be seen from the higher Qe values in Figure 3.1, the wood chars amended with
aluminum oxides (Al wood char) are much more effective at fluoride removal at all
charring temperatures than unamended wood char even at the highest charring
temperature (and thus highest specific surface area). Figure 3.1 shows that Qe values go
from highest to lowest along with charring temperature in the order of 600 °C > 500 °C
> 400 °C > 300 °C. Based on the data in italics in Table 3.1, the bottom four rows
where the 1/n constants have been forced to a common value to allow for direct
comparison of Kf, it is clear that the eucalyptus wood char amended with aluminum
54
oxides is more effective at removing fluoride than unamended wood char. When
comparing wood chars with and without aluminum amendment at 600° C the difference
in fluoride removal results is statistically significant at p = 0.05.
3.5
600 °C Aluminum WC
500 °C Aluminum WC
3
400 °C Aluminum WC
2.5
300 °C Aluminum WC
600 °C Wood Char (WC)
Qe (mg/g)
2
1.5
1
0.5
0
0
20
40
60
80
100
Ce (mg/L)
Figure 3.1: The equilibrium fluoride concentration, Ce, in mg/L (x-axis) versus mg of
fluoride removed per gram of material, Qe, (y-axis) for both aluminum amended and
unamended eucalyptus wood chars.
Specific Surface Area
Brunson and Sabatini (2009) showed that charring temperature has a marked
effect on the specific surface area and the fluoride adsorptive capacity of bone char.
Other researchers have confirmed that the specific surface area of wood chars changes
as a result of charring temperatures (James et al., 2005; Zhou et al., 2010). The results
of this work show that for eucalyptus wood char a similar trend can be seen where
55
charring temperature affects both surface area and fluoride adsorption capacity. Table
3.2 and Figure 3.2 show that charring eucalyptus wood has a pronounced effect on the
specific surface area of the resulting chars and that specific surface area increases as
charring temperature increases. Abe et al. (2004) suggested that fluoride adsorption by
carbonaceous materials improves with increasing specific surface area and that it is
dependent on pore size distribution. The data for wood char matches the trend shown by
Table 3.2: Specific surface area and pHPZC of aluminum amended, reducing agent
modified and unamended eucalyptus wood and bone chars at various charring
temperatures.
Treated and Untreated Wood and Bone Char Parameters
Charring Temperature/
Pretreatment
300 °C
Material
Amendment
None
Specific Surface
Area (m2/g)
0.90 ± 0.1
Wood Char
5.8 ± 0.2
400 °C
Wood Char
None
13.5 ± 1.3
7.2 ± 0.5
500 °C
Wood Char
None
149 ± 2.6
8.9 ± 0.2
600 °C
Wood Char
None
327 ± 7.0
9.6 ± 0.4
500 °C
Bone Char
None
99.1 ± 0.8
8.2 ± 0.4
300 °C
Wood Char
Aluminum
0.70 ± 0.1
4.0 ± 0.2
400 °C
Wood Char
Aluminum
17.1 ± 0.3
4.3 ± 0.1
500 °C
Wood Char
Aluminum
151 ± 6.4
5.5 ± 0.4
600 °C
Wood Char
Aluminum
256 ± 9.1
5.7 ± 0.2
500 °C
Bone Char
Aluminum
93.2 ± 6.8
7.0 ± 0.1
(NH4)2Fe(SO4)2/600 °C
Bone Char
None
94.1 ± 3.0
7.5 ± 0.1
(NH4)2Fe(SO4)2/600 °C
Wood Char
None
319 ± 3.6
8.0 ± 0.6
Na2S2O4/600 °C
Bone Char
None
96.0 ± 1.2
6.8 ± 0.2
Na2S2O4/600 °C
Wood Char
None
315 ± 3.3
6.8 ± 0.6
56
pHPZC
Abe et al. (2004) - while virtually no fluoride removal was observed for wood charred
at 300 °C with a specific surface area of 0.90 m2/g, the adsorption increased slightly at
500 °C (0.01 mg/g at Ce=10 mg/L) where the specific surface area was 149 m2/g and the
highest fluoride removal capacity was achieved at 600 °C (0.11 mg/g at Ce=10 mg/L)
with a surface area of 327 m2/g. Nonetheless, the fact that the wood char was minimally
effective at fluoride adsorption regardless of charring temperature or surface area
suggests that the surface chemistry of wood char is not conducive to attracting
negatively charged fluoride ions.
350
10
Specific
surface area
unamended
wood char
9
300
250
7
6
200
5
150
4
3
100
2
50
pH Point of Zero Charge
Specific Surface Area (m2/g)
8
Specific
surface area
amended
wood char
pH point of
zero charge
unamended
wood char
1
0
0
300 °C
400 °C
500 °C
600 °C
Charring Temperature
pH point of
zero charge
amended
wood char
Figure 3.2: Bar graph shows specific surface area (m2/g) (left y-axis) and line graph
shows the pHPZC (right y-axis) while charring temperature for both aluminum amended
and unamended eucalyptus wood chars is shown on the x-axis.
57
Wood chars amended with aluminum oxides exhibit similar specific surface
areas to those of the unamended chars and show the highest surface areas at 600 °C and
the lowest at 300 °C. The low level of aluminum oxides incorporated into the media
during treatment (approximately 0.25% aluminum by weight) was enough to greatly
increase the adsorption capacity of the media but not to cause a significant change in the
specific surface area (some samples, 600 °C, show a slight decrease while others, 400
°C, do not). Any decrease in specific surface area due to amendment with aluminum
oxides is offset by the improved surface chemistry as noted by the large increase in the
fluoride removal capacity. The fact that the 500 °C bone char has a lower surface area
(99 versus 327 m2/g) relative to the 600 °C wood char and yet evidences a much higher
fluoride adsorption supports the suggestion that the chemical composition, with its
resulting change in surface chemistry, of adsorption materials is important as discussed
above.
Point of Zero Charge
Another property of interest is the pHPZC, a characteristic of media assessed to
determine the surface charge at different water pH levels. Figure 3.2 and Table 3.2
show that both charring temperature and aluminum amendment affect the pHPZC. For
example, at 300 °C the pHpzc of wood char was 5.8 while at 600 °C it was 9.6. When
the aluminum amendment is taken into account, results show that at 600 °C the pHpzc
values are 9.6 and 5.7 for unamended and amended, respectively (Table 3.2).
Chingombe et al. (2005) stated that thermal treatment of activated carbon based
materials has been known to produce more basicity on the surface of media. In this
58
case, a higher charring temperature resulted in a higher pHPZC, which is likely due to the
removal of oxygen atoms on the surface that in turn reduced the acidic functional
groups and raised the pHPZC (Hao et al., 2004). Work done by other authors has shown
the pHPZC of activated carbon-based materials to fall in the range of 9 – 10 (Han et al.,
2000; Song et al., 2010; Gonzalez and Pliego-Cuervo, 2013). Research on bone char
also supports this finding that higher charring temperatures tend to increase the pHPZC
or residual pH at equilibrium (Larsen et al., 1994; Mwaniki and Nagelkerke, 1990). It
is interesting to note, as shown in Table 3.2, that the pHPZC of the wood char actually
decreases when chars are amended with aluminum oxides. The fact that the pHPZC
decreases to below 6 is a surprising result given the increased fluoride removal.
However, this is a feasible result as the pHPZC values of aluminum oxides range from
4.5 to 9.8 (Smit and Holten, 1980; Sposito, 1996). A decrease in pHPZC also results in a
decrease in the pH of solution at equilibrium when fluoride is removed with aluminum
amended wood char. Residual solution pH values for untreated wood char at 600 °C
averaged 7.38 while the aluminum amended wood char averaged 6.18. This decrease in
solution pH value through use of aluminum amended wood char may have played a role
in the fluoride adsorption capacity since some pH values fell below the pHPZC.
Surface Modification
Activated carbon surfaces can exhibit acidic, basic, or neutral characteristics
depending on the surface functional groups; treatment with oxidizing or reducing agents
can alter the surface functional groups and thus surface characteristics such as the pHPZC
of media (Daifullah et al., 2007; Liu et al., 2007; Song et al., 2010). Table 3.2 shows the
59
specific surface area and pHPZC of chars treated with reducing agents, sodium dithionite
(Na2S2O4) and iron ammonium sulfate [(NH4)2Fe(SO4)2]. It was hypothesized that
treatment with reducing agents would increase the pHPZC by making the surface of the
media more basic, likely through removal of oxygen atoms. This work did not confirm
the hypothesis. For example, when wood char was treated with iron ammonium sulfate
the pHPZC of the wood char decreased from 9.6 to 8.0. Additionally, treatment with
sodium dithionite evidenced even larger changes with both eucalyptus wood and bone
chars exhibiting a final pHPZC of 6.8, much lower than their starting values of 9.6 and
8.2, respectively (see Table 3.2 for pHPZC values). Han et al. (2000) found that treatment
with strong reducing agent sodium dithionite also did not improve the chromium
removal capacity of activated carbon. They speculated that the activated carbon surface
may have a buffering capacity, due to its ability to oxidize reducing agents without
significant changes in the media surface, that makes it resistant to change from reducing
agents. This may, in part, explain why the pHPZC was not altered in the expected
direction in the case of the eucalyptus char.
Despite the decrease in pHPZC between untreated wood and bone chars and chars
treated with reducing agents, the equilibrium solution pH values were similar for both
untreated and pretreated. For example, solution pH values after reaching equilibrium
with wood and bone chars alone were in the range of 7.09 to 7.75 and 7.33 to 7.74,
respectively, while for all chars treated with reducing agents the equilibrium solution
pH values ranged from 7.15 to 7.69. The Kf values for fluoride removal with amended
materials shown in Table 3.3`can be directly compared because the 1/n values were
forced to the same value for each material. These Kf values demonstrate that for
60
unamended wood and bone chars, the use of reducing agents increased the fluoride
removal capacity minimally (e.g., the Kf value for bone char increased from 1.82 to
2.28 when treated with sodium dithionite) despite the decrease in pHPZC, 8.2 to 6.8 for
bone char. Treatment with reducing agents caused no significant change to specific
surface area of materials.
Therefore, this change in fluoride removal cannot be
explained by removal of organics, significant change in solution pH, or increase in
surface area.
Table 3.3: Freundlich constants calculated from batch tests investigating fluoride
removal of bone and wood chars modified with reducing agents and amended with
aluminum or unamended. In all cases 1/n values have been forced to a common values
so Kf constants can be statistically compared.
Freundlich Constants
Charring
Temp.
Pretreatment
Amendment
Material
Kf
(mg/g)/(mg/L)1/n
1/n**
r2
600 °C
None
None
Wood Char
0.04 ± ≤0.01
0.50
0.91
600 °C
Na2S2O4
None
Wood Char
0.05 ± ≤0.01
0.50
0.96
600 °C
(NH4)2Fe(SO4)2
None
Wood char
0.05 ± ≤0.01
0.50
0.99
600 °C
None
Aluminum
Wood Char
0.18 ± ≤0.01
0.64
0.99
600 °C
(NH4)2Fe(SO4)2
Aluminum
Wood Char
0.09 ± ≤0.01
0.64
0.99
500 °C
None
None
Bone char
1.82 ± 0.09
0.33
0.97
500 °C
Na2S2O4
None
Bone Char
2.28 ± 0.25
0.33
0.97
500 °C
(NH4)2Fe(SO4)2
None
Bone Char
2.23 ± 0.10
0.33
0.98
500 °C
None
Aluminum
Bone char
5.31 ± 0.18
0.40
0.99
500 °C
(NH4)2Fe(SO4)2
Aluminum
Bone Char
2.29 ± 0.09
0.40
0.98
**For this data the 1/n Freundlich values have been forced to a common value so that the K values can be directly compared. Due
f
to process of forcing the 1/n values to a common number there is no error for these 1/n values.
61
Therefore, an alternative possibility is that while not increasing the pHPZC, the treatment
with strong reducing agents altered the surface functional groups on the chars in a way
that made the surface more attractive to the negatively charged fluoride. Conversely,
Table 3.3 shows that when bone and wood char were pretreated with reducing agents
and then amended with aluminum sulfate, the fluoride removal actually decreased as
compared with aluminum amendment but no pretreatment (e.g. Kf for iron ammonium
sulfate reduced aluminum modified bone char was 2.29 while the Kf for aluminum
amended bone char was 5.31). This decrease in fluoride removal may be due to the
decrease in acidic functional groups caused by treatment with reducing agents which
would in turn decrease media uptake of aluminum and therefore, reduce fluoride
removal from the aqueous solution. Song et al. (2010) found that pretreatment with
oxidizing agents added acidic functional groups to the surface which enhanced the
attraction of an activated carbon-based material to lead, another metal. Therefore,
removing acidic functional groups from the surface should decrease the attractive forces
between aluminum oxides and the material surface.
These findings suggest that
pretreatment of chars with reducing agents as tested here is not a helpful method for
increasing fluoride removal capacity.
Comparison between Wood and Bone Chars
A motivation for this work was to find an alternative to using bone char for
fluoride removal that may be more acceptable in select communities. In this research,
eucalyptus wood char has proven to be a viable fluoride removal option only if it is
62
amended with aluminum oxides. Despite the high specific surface area, the wood char
requires amendment to approach the removal capacity of the bone char. Chemical
composition, hydroxyapatite versus carbon for bone and wood chars, respectively, was
discussed above as one of the key differences between the two chars. Due to the limited
fluoride removal of carbon alone, the wood char benefits from the addition of aluminum
oxides which provides a mechanism by which fluoride forms a bond with the aluminum
present on the char, likely through ion exchange, motivating the adsorption. Another
factor which could cause an adsorption difference between bone and wood chars is pore
size distribution. If pore size distribution is different for bone and wood chars it could
cause size exclusion when adsorption of fluoride or aluminum oxides occurs in the
material with smaller pores. However, in this case, except for wood charred at 600 ˚C,
all the pores in both wood and bone chars were found to be mesopores (20 - 500 Å).
The wood charred at 600 ˚C had 40% of its total pore size as micropores (<20 Å).
Therefore, since the hydrated radii of aluminum and fluoride have been reported to be
4.75 and 3.52 Å, respectively (Nightingal, 1959), no size exclusion of these dissolved
ions should have occurred. Nonetheless, the size of colloidal aluminum oxide
precipitates would be orders of magnitude higher and thus could experience size
exclusion in the smaller pores. It is anticipated that the micropores present in the 600 ˚C
char explain why it was the only temperature at which the aluminum amendment caused
a significant change to the specific surface area. This is due to the aluminum oxides
blocking some of the micropores during the amendment process. The pHPZC values of
both materials are diverse as unamended bone char is 8.2 while unamended wood char
is 9.6 and the aluminum amended versions are 7.0 and 5.7 for bone and wood,
63
respectively (shown in Table 3.2). This suggests that while the pHPZC is a helpful
characteristic to understand about media surfaces, it is not the only indicator of the
fluoride removal capacity. This research demonstrated that despite the pHPZC values for
bone char and aluminum amended wood char being lower than those of other materials,
the fluoride removal capacities were higher.
Conclusion
Although bone char has been proven as an effective fluoride adsorption material
due to surface chemistry and specific surface area, in some areas it may not be suitable
for use to remove fluoride from drinking water based on cultural reasons. Therefore,
this work assessed the potential for eucalyptus wood char to be a substitute adsorptive
material for bone char. In spite of the fact that the wood char exhibited higher specific
surface area values at charring temperatures of 500 and 600 °C than bone char, it
evidenced negligible fluoride removal capacity due to the unfavorable surface chemistry
and chemical composition. Amending the wood char with aluminum oxides
significantly increased fluoride uptake – this material having both desirable surface area
and surface chemistry. The pHPZC of the wood char increased with higher charring
temperatures, which is likely due to the increased basicity of the surface after charring
at higher temperatures. When amended with aluminum nitrate, the wood char, due in
part to the strong attraction between aluminum and fluoride in water, exhibited a
statistically significant increase in fluoride removal capacity despite the decrease in the
pHPZC. Unstudied until now, treatment with reducing agents, intended to raise the
pHPZC, actually lowered the pHPZC but did not greatly alter the equilibrium pH values in
64
solution. Treatment with reducing agents, while minimally increasing the fluoride
removal capacity of unamended chars, did not cause enough change to make this a
useful research area to pursue further at this time. Helpful to this line of inquiry towards
finding a suitable substitute for bone char would be an investigation into other methods
of amending wood chars with positively charged metals, such as iron and aluminum, to
see if it is possible to get a higher percentage metal loading on the media and in doing
so to increase the fluoride removal capacity. Additionally helpful would be to assess
whether these types of surface area and surface chemistry amendments are equally or
even more effective for other types of locally available materials such as bamboo,
coconut waste, coffee production waste, or other wood chars.
Acknowledgements
The authors gratefully acknowledge the U.S. Environmental Protection Agency
and the U.S. National Science Foundation that contributed partial funding to this work.
This publication was developed under STAR Fellowship Assistance Agreement no.
91731301 awarded by the U.S. Environmental Protection Agency (EPA). This work has
not been formally reviewed by EPA, and the views expressed in this paper are solely
those of the authors and the EPA does not endorse any products or commercial services
mentioned in this publication. The authors also wish to acknowledge partial funding
from the OU WaTER Center and the University of Oklahoma in support of this work.
65
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71
CHAPTER 4
Practical Considerations, Column Studies and Natural Organic
Material Competition for Fluoride Removal with Bone Char and
Aluminum Amended Materials in the Main Ethiopian Rift Valley.3
Abstract
This work investigated the fluoride removal capacities of materials including:
bone char (BC), aluminum oxide coated bone char (ACBC) and aluminum oxide
impregnated wood char (AIWC) and activated alumina (AA).
Materials were
investigated in batch and column studies, including comparisons between synthetic and
natural groundwater. Results suggest that in all cases the laboratory column results
exhibited higher fluoride removal efficiencies than the field studies conducted in the
Ethiopian Rift Valley. Further studies indicate that the reduced effectiveness in the
field was likely due to a combination of the high pH of the groundwater (8.2) and the
presence of competing ions (sulfate). Batch studies testing potential competition with
natural organic material (NOM) showed no statistical evidence of competition for
adsorption on BC between fluoride and NOM and minor evidence of competition when
using ACBC and AIWC. To provide evidence for using Rapid Scale Small Column Test
(RSSCT) principles for fluoride adsorption by BC, two different column volume and
3 Brunson, L.R., and Sabatini, D.A. (2014). “Practical considerations, column studies and natural organic
material competition for fluoride removal with bone char and aluminum amended materials in the Main
Ethiopian Rift Valley.” Sci. Tot. Environ. 488-489, 580–587.
72
particle sizes were tested. The results indicate that RSSCT scaling equations, developed
for activated carbon, are applicable for BC removal of fluoride. These results provide
valuable insights for utilizing fluoride adsorption materials for water treatment in a field
setting.
Introduction
Lack of access to an improved source of drinking water is a global health issue
affecting approximately 780 million people worldwide (WHO and UNICEF, 2012). Not
considered in this statistic are an estimated 200 million people who potentially drink
groundwater containing fluoride concentrations above the World Health Organization
(WHO) guideline value of 1.5 mg/L (Amini et al., 2008; World Health Organization,
2011). Elevated fluoride concentrations occur in numerous groundwater formations
globally, including areas of India, China, and the Rift Valley of Africa which includes
the Main Ethiopian Rift Valley (MER) (Amini et al., 2008; Fawell et al., 2006;
Fewtrell et al., 2006; Rango et al., 2010, 2012).
Human
consumption
of
elevated
fluoride concentrations is problematic because dental and skeletal fluorosis can result
(Li et al., 2001; Meenakshi and Maheshwari, 2006; Sivasamy et al., 2001).
Dental
fluorosis manifests itself as discoloration and pitting of the teeth, resulting in physical
and economical effects, while skeletal fluorosis can cause pain and restriction of
movement, which can impair the livelihood of those dependent on subsistence farming
or other manual activities (Fewtrell et al., 2006; Meenakshi and Maheshwari, 2006;
Tonguc et al., 2011).
73
When harvesting rainwater or using alternative water supplies is not practical,
fluoride removal from drinking water is the most viable option for obtaining safe water.
Treatment options that have been investigated include: electrolytic defluoridation
(Gwala et al., 2011; Mameri et al., 2001), reverse osmosis and membrane processes
(Arora et al., 2004; Meenakshi and Maheshwari, 2006), precipitation (Yadav et al.,
2006), and adsorption (Abe et al., 2004; Ayoob et al., 2008; Brunson and Sabatini,
2009; Medellin-Castillo et al., 2007; Mlilo et al., 2010). Many of these options have
proven viable for fluoride removal, but also exhibit aspects which make them less than
ideal for rural developing areas. For example, electrolytic defluoridation requires an
energy source, and reverse osmosis and membrane processes can require large capital
costs and produce large volumes of waste solution (Gwala et al., 2011;
Jagtap et al., 2012). The Nalgonda technique is a process by which aluminum sulfate
and lime are added to water to cause co-precipitation of fluoride and aluminum
hydroxides. Nalgonda has been used with some success, including in Ethiopia, but has
constraints including the production of large amounts of waste sludge and the
requirement of daily operational and chemical addition (Fawell et al., 2006; Meenakshi
and Maheshwari, 2006; Yadav et al., 2006).
Adsorption is a helpful fluoride removal option because it can be implemented
using gravity flow (no need for a power source), and is helpful in terms of cost and ease
of use (Bhatnagar et al., 2011). Researchers have extensively studied activated alumina
(AA), a material well known for fluoride adsorption. It has been found that AA works
best at low pH values (< 5) and that it has a fluoride removal capacity ranging from 4 15 mg of fluoride per gram of AA for laboratory studies and 1 mg/g in a field study;
74
fluoride removal values for AA tend to vary based on water pH, competing ions and the
nature of the study (Fawell et al., 2006; Ghorai and Pant, 2005; Hao and Huang, 1986;
Tang et al., 2009). A number of activated carbon materials showed minimal fluoride
removal capacities despite their high specific surface areas (Abe et al., 2004; Ayoob et
al., 2008). Bone char (BC) has demonstrated a much higher fluoride removal capacity
than activated carbon, which can be attributed to its dual properties of a high specific
surface area [111 m2/g (Brunson and Sabatini, 2009), 100 m2/g (Cheung et al., 2005)
and 104 m2/g (Medellin-Castillo et al., 2007)] and desirable surface chemistry, a surface
that exhibits a net positive charge at typical groundwater pH values so that it will attract
fluoride ions. Researchers have shown that BC can remove fluoride and arsenic
simultaneously with minimal competition, which is helpful in situations where they
coexist; several MER locations have been suggested to have both fluoride and arsenic in
the groundwater (Rango et al., 2010). Further, BC can be produced using local materials
in rural areas of Ethiopia, Kenya and Tanzania (Abaire et al., 2009; Brunson and
Sabatini, 2009; Mjengera and Mkongo, 2002). In some cases, BC is culturally
unacceptable, therefore, alternative materials with similar properties to BC must be
evaluated.
When considering the practicalities of removing fluoride from groundwater it is
necessary to account for possible competition from other ions in water. Often phosphate
and sulfate in groundwater compete with fluoride for adsorption by commonly used
adsorbents. (Jagtap et al., 2012). Natural organic material (NOM) in the groundwater
should also be considered as it sometimes competes with other adsorptive processes,
particularly adsorption onto activated carbon, and is found in all groundwater sources
75
(Newcombe et al., 1997), generally at concentrations of 0.5 – 10 mg/L (Genz et al.,
2008). NOM is composed of humic and fulvic acids and typically has negatively
charged surface functional groups (Redman et al., 2002), which suggests it has the
potential to compete with fluoride for removal on adsorption media in the same way
that negatively charged phosphate and sulfate ions are known to compete. It does not
appear that NOM competition has previously been studied in relation to fluoride
adsorption onto bone or wood chars and competing ion studies have not been conducted
on several of the materials studied in this work.
Continuous flow column studies are important to assess the ability of a sorbent
to remove fluoride in a setting similar to a point-of-use (POU) or community scale
system. Continuous flow tests capture kinetic effects, such as mass transfer and
intraparticle diffusion, in a way not possible in batch tests. Ma et al. (2008) conducted
batch and column studies and suggested that BC is more effective than AA at fluoride
removal and that reducing the bed depth or increasing the flow rate of a column study
decreased the bed volumes until breakthrough.. Minimal work to date has evaluated the
fluoride removal potential of bone and wood chars in continuous flow studies,
particularly in a field setting utilizing a natural water source (Ayoob and Gupta, 2007;
Kloos and Haimanot, 1999).
Assessing filtration technologies for use in community scale systems can be
costly and time consuming. One way to reduce these issues is through the use of Rapid
Small Scale Column Test (RSSCT) principles which are helpful for reducing the
amount of water and media required, the time needed to conduct studies, and the cost of
such studies while producing results representative of full scale systems (Crittenden et
76
al., 1986; Badruzzaman et al., 2004). RSSCT calculations use dimensionless scaling
principles based on mass transfer and hydraulic flow conditions as in Equations 4.1 and
4.2, where EBCT is the empty bed contact time, d is the particle diameter of the sorbent,
ν is superficial velocity, and SC and LC stand for small-scale and large-scale,
respectively (Badruzzaman et al., 2004; Trussell et al., 2005).
EBCTSC/EBCTLC = [dSC/dLC]^2
Equation 4.1
ν SC/ ν LC = dLC/dSC
Equation 4.2
RSSCT equations were used to calculate particle size, flow rate and bed
volumes for use in two column studies to evaluate the validity of this approach for the
use of BC for fluoride removal (Trussell et al., 2005). The use of these equations for
the RSSCT approach assumes that BC is homogenous at different particle diameters and
that surface diffusivity is constant across different particle sizes (Westerhoff et al.,
2005). RSSCTs can be valuable in scaling from small scale laboratory column systems,
that can be tested in a few weeks, to full scale systems, which require multiple months
(Badruzzaman et al., 2004; Westerhoff et al., 2005). Theoretically, if the RSSCT model
is applicable, two columns of different sizes that are accurately scaled should have the
exact same breakthrough curves, all other things equal (Westerhoff et al., 2005). To
date the RSSCT method, first developed for activated carbon and later validated for
arsenic removal on iron oxide based media, based on reviewed literature, has not been
evaluated for BC adsorption of fluoride (Westerhoff et al., 2005).
The purpose of this work is to assess the ability of several materials (wood char,
aluminum coated wood char, aluminum impregnated wood char, bone char, and
aluminum coated bone char) to remove fluoride in a continuous flow system utilizing
77
groundwater with elevated fluoride concentrations from the MER. Continuous flow
tests will also be conducted in a laboratory setting to compare the relative fluoride
removal capacities between laboratory and field systems. There are two hypotheses
being tested in this work. The first is that, based on the relatively homogenous nature of
the BC material, the RSSCT approach will be validated for BC, which would allow for
quicker and easier scale up of systems intended to remove fluoride from water using
bone char. This work also aims to assess the practical question of whether or not NOM
will negatively affect the fluoride removal capacity of several materials. Thus, the
second hypothesis is that NOM will have an effect on fluoride removal due to its
negative charge.
Methods
Materials Preparation
Eucalyptus robusta was obtained from Mezozoic Landscapes, Inc. in Florida
while fish bone meal was purchased from Peaceful Valley Farm & Garden Supply in
California. Chars were prepared by placing raw material in a glaze-free covered
porcelain container and charring them in an oven at 500 or 600 °C, bone and wood,
respectively, for four hours. Materials were crushed and sieved to a size range of 180 –
425 μm in diameter and rinsed in deionized water to remove fines. Aluminum oxide
coating was achieved by first using 4-Morpholineethanesulfonic acid hydrate, a
biological pH buffer, in deionized water to obtain a solution pH of 3.5. Aluminum
nitrate was then added at 175 mg/L along with 25 grams of char material, based on the
work of Levya-Ramos et al. (1999), to 500 mL flasks. This mixture was shaken for 5
78
days and then char materials were filtered, rinsed, dried at 100 °C and stored in sealed
plastic bags. Aluminum oxide impregnated wood char (AIWC) was prepared based on
the work of Tchomgui-Kamga et al. (2010) where small pieces of raw eucalyptus wood
were boiled for 1.5 hours in a 1 molar aluminum chloride solution. Char was then left
to cool in solution for 2 hours, air-dried for 2 hours, and oven dried at 100 °C. Dried
wood pieces were charred at 600 °C for 1 hour, crushed, sieved, rinsed, dried and stored
as described above.
Analysis
Fluoride in the field was analyzed with an Extech Pocket Fluoride Tester
(FL700) portable fluoride meter using Total Ionic Strength Adjustment Buffer (TISAB)
reagent tablets. The Extech Pocket Fluoride Tester has an accuracy of ± 3% and a range
from 0.1 to 9.99 mg/L. Fluoride and pH analyses in the laboratory were conducted
using an Orion pH/ISE meter (model 710a) and fluoride was measured using equal
amounts of sample and TISAB.
Qe = KfCe1/n
Equation 4.3
Qe = (CeQmb)/(1+Ceb)
Equation 4.4
Freundlich and Langmuir isotherm equations (Equations 4.3 and 4.4, respectively) were
used to analyze the batch test data, where Ce is the fluoride concentration at equilibrium,
Qe is the adsorption of fluoride per gram of sorbent, Kf is the adsorption capacity, 1/n is
the adsorption intensity, Qm is the maximum fluoride adsorption per gram of media and
b is a measure of adsorption affinity (Chen et al., 2011; Leyva-Ramos et al., 2010; Ma
79
et al., 2008). SigmaPlot Version 11.0 software was used to perform regression analyses
and statistical assessments of the data fitted to these two models.
Batch Studies
Batch tests were conducted by placing 0.5 grams of media in 250 mL HDPE
Nalgene bottles with 50 mL of prepared fluoride solution. Sodium fluoride was used to
make a 1,000 mg/L stock solution which was diluted with deionized water containing
0.05 molar sodium chloride (to provide a constant ionic strength) as needed to make
solutions. Initial batch concentrations ranged from 2.5 - 100 mg/L fluoride. NOM was
added to select batch studies as either Pahokee Peat humic acid (PPHA) or Nordic
Aquatic fulvic acid (NAFA), purchased from the International Humic Substances
Society, selected due to the fact that they are representative of a range of naturally
occurring NOM. Sodium sulfate or organic buffers (MES, HEPES, TAPS), to test for
competition or adjust the pH, respectively, were added to select systems. Sealed bottles
were shaken at 200 shakes per minute for 24 hours to reach equilibrium. At that point,
solutions were allowed to settle, then filtered using Whatman 11 micron filters to
remove media, and tested for fluoride concentrations and pH values.
Column Tests
All column studies, with one exception, were conducted utilizing a glass column
size 1 x 10 cm (empty bed volume of 7.9 mL) with a flow rate of 1.2 mL/min. While,
due to availability, the particle diameter for AA was 125 μm, the diameter for the other
materials was an average of 302 μm. In order to test RSSCT principles with BC, a
80
second BC column was run using different column and paticle sizes. This study used a
glass column size 2.5 x 7.1 cm (empty bed volume of 34.9 mL) with an average media
size of 637 μm selected based on Equation 4.1. In this case, in order to avoid high head
loss often experienced with high superficial velocities, Equation 4.2 was validated for
use by verifying that the product of the Reynolds and Schmidt numbers was in the range
of 160 to 40,000, which is acceptable in cases where dispersion is not the primary
mechanism (Trussell et al., 2005). At the base of the column, glass beads and glass
wool were placed to enhance even solution distribution and keep the media contained,
with the same arrangement in reverse above the column. Water was pumped through the
column from the bottom at approximately 1.2 mL/min using variable-flow peristaltic
pumps from Fischer Scientific. Columns were packed with wet media to ensure that
media was uniformly packed in the column without an excess of air. Samples were
taken periodically throughout the column run and tested for pH and fluoride
concentration until complete saturation (Ceffluent = Cinfluent) occurred. For field studies
water was collected from a community in the MER near Meki, Ethiopia. For laboratory
studies, synthetic fluoride solutions were made using deionized water and fluoride. In
select cases, the water was pH adjusted to 8.2 using TAPS organic buffer or 250 mg/L
of sodium sulfate was added.
Results and Discussion
Screening of Media for Column Studies: Batch Results
Batch isotherms were used to compare the removal capacities of novel materials
in order to select the best sorbent for column studies. Five materials were considered in
81
the batch tests including: wood char (WC), aluminum oxide coated wood char
(ACWC), aluminum oxide impregnated wood char (AIWC), bone char (BC), and
aluminum oxide coated bone char (ACBC). Adsorption isotherm data for these five
novel materials and activated alumina (AA) are shown in Table 4.1.
Table 4.1: Qe data for six media evaluated in batch isotherm studies
Wood Char
<0.1 ± 0.0
0.82
NA*
Qe
(mg/g) at
Ce= 1.5
mg/L F
0.2
Al Coated Wood Char
0.2 ± 0.0
0.69
NA*
0.2
0.7
Bone Char
1.8 ± 0.2
0.38
6.1 ± 0.4
2.1
3.9
Aluminum Coated Bone Char
1.4 ± 0.1
0.42
11.4 ± 0.9
1.7
3.5
Aluminum Impregnated Wood Char
4.6 ± 0.6
0.61
16.8 ± 5.8
6.0
18.8
Activated Alumina (AA)
0.4 ± 0.1
0.48
4.1 ± 0.9
0.5
1.2
Material
Kf
(mg/g)(mg/L)1/n
1/n
Langmuir
Qm (mg/g)
Qe
(mg/g) at
Ce= 10
mg/L F
0.1
*NA – not applicable – the data did not approach a plateau thus making the Langmuir isotherm inappropriate.
Note: R-squared values for all Freundlich and Langmuir data in this table are in the range of 0.95 to 0.99
For WC and ACWC only Freundlich parameters are shown, as the isotherms exhibited
such low removal capacities that a plateau was not exhibited and thus, Langmuir
interpretation is not appropriate. The Freundlich isotherm can be indicative of a
heterogeneous material and has been used in the literature to evaluate coal-based
sorbents, BC, and pine wood char (Brunson and Sabatini, 2009; Mohan et al., 2012;
Sivasamy et al., 2001). In looking at the results in Table 4.1, Kf values should be
compared only if 1/n values are equal. Since values in Table 4.1 show a wide range of
values for 1/n, the Freundlich equation is used to calculate Qe values at Ce = 1.5 and Ce
= 10 mg fluoride/L to compare the adsorption capacity at a constant Ce value (driving
force for adsorption). For remaining materials the Langmuir isotherms were calculated
and Qm data, which suggest the maximum adsorption capacity of the material for that
sorbate, are compared. AA, BC, ACBC, and AIWC were selected for further study.
82
Column Studies
83
84
deionized water. B. Field studies using native groundwater with an influent pH of approximately 8.2
concentrations in mg/L. A. Lab studies where the influent pH (unadjusted) was approximately 6.0 and the solution was fluoride in
Figure 4.1: Results of continuous flow columns studies where the x-axis shows bed volumes and the y-axis shows effluent fluoride
Fluoride removal capacities were evaluated in laboratory column studies.
Investigation of Figure 4.1 and Table 4.2 offers several interesting findings. The first
observation is that the same sorption trends shown in the batch studies are also shown in
laboratory column studies: AIWC > ACBC > BC > AA (compare bed volumes to reach
1.5 mg/L in Figure 4.1(A) and Table 4.2 with Qm values in Table 4.1). It is interesting
to note the variation in specific surface areas for these materials as the AIWC was found
to be 284 ± 5.9 m2/g, whereas BC and ACBC are 99.1 ± 0.8 and 91.8 ± 3.0 m2/g,
respectively, and the AA is 185 m2/g based on manufacturer information. This higher
specific surface area could indicate one key reason why the AIWC exhibits a larger
fluoride removal capacity than the other media. All columns were run with the same
bed volume size and volume of column output was measured, therefore it is feasible to
compare fluoride removal capacities based on bed volumes across materials.
A mass balance of the columns was conducted to determine the weight of
fluoride adsorbed per gram of media at exhaustion (when Ceffluent = Cinfluent = 8.6 mg/L);
these values are summarized in Table 4.2. In all cases, except for AA, the column study
Qexhaustion values (Ceff = Cinf) were found to be less than the batch Langmuir Qm, which is
expected since Qm can occur at Ce values much higher than the Ce = 8.6 mg/L selected
as representative of groundwater in the MER. This is consistent with other findings in
the literature that compared batch and column studies such as Ayoob and Gupta (2007)
who found that for alumina cement granules being used to remove fluoride, the Qcolumn
was 5.90 mg/g while the Langmuir Qm was 10.22 mg/g.
85
Table 4.2: Qe in mg of fluoride removed per gram of material calculated based on the
amount of fluoride removed during the continuous flow column study run to saturation
(Ceffluent = Cinfluent). The bed volumes to Ceff = 1.5 mg/L were obtained using the data
presented in Figure 4.1. The Langmuir equation was used and both the Langmuir Qm
and the Qe value at 8.6 equilibrium fluoride concentration are presented.
Column
Bed Volumes to
Reach 1.5 mg/L
Fluoride Effluent
Batch
Column
Mass Balance
QexhaustionValues
Cinf = 8.6 mg/L
Langmuir
Qm (mg/g)
Lab
Fig 1(A)
Field
Fig 1(B)
Activated Alumina
4.1
70
135
5.9
1.2
Bone Char
6.1
100
143
5.7
3.0
Aluminum Coated Bone Char
11.4
187
180
6.3
4.8
Aluminum Impregnated Wood Char
16.8
253
89
8.3
1.7
Material
Lab
(mg/g)
Field
(mg/g)
In both laboratory and field column studies (Figure 4.1A and 4.1B, respectively)
the breakthrough curves of the AIWC and the AA are steeper than the other media,
which suggests they have smaller mass transfer zones (less diffusion limited). For the
AA this is attributed to its particle size (125 µm), which is smaller than all the other
studied media. Smaller particle size results in shorter intraparticle diffusion distances
and thus, a smaller mass transfer zone; nonetheless its lower fluoride removal capacity
causes its breakthrough curve to occur earlier. The other materials all had the same
particle size (approximately 302 µm), which suggests particle size is not the cause of
the steeper breakthrough curve for AIWC. Instead, the sharp breakthrough curve of the
AIWC can potentially be attributed to less mass transfer limitations due to intraparticlelimited diffusion - reasons for this are unclear at this time, but one possibility is that
most of the adsorbing surfaces are near the outer region of the sorbent, which would
86
reduce the amount of internal diffusion required. It should be noted that in the case of
the laboratory results for AIWC, the breakthrough curve is still sharp, but the material
showed the largest amount of bed volumes at low fluoride concentrations before the
relatively steep breakthrough curve occurred, thus making it the most successful
material at meeting the WHO standard.
In all cases, the Qexhaustion values calculated from the continuous flow data were
higher in the laboratory setting than in the field setting (Table 4.2). This is expected
due to constituents in the natural groundwater that can reduce adsorption; e.g. higher pH
values, the potential for competing ions, and possible competition from NOM in the
groundwater. Each potential effect is evaluated in a subsequent section. These findings
are consistent with those of Kamble et al. (2007) who found that fluoride removal with
another media (lanthanum-modified chitosan) was significantly higher when tested with
distilled water and fluoride, than when field water samples were used; they attributed
this to the pH (approximately 7) and the presence of competing anions in the field
water. The BC and ACBC both show a similar percentage change (less than 65%) in
Qexaustion between field and laboratory results of (e.g., 5.7 for lab and 3.0 mg/g for field
for BC), which is similar to the approximately 67% difference shown by
Kamble et al. (2007) for lanthanum-modified chitosan at an equilibrium value of 8
mg/L between field and synthetic samples. However, the AA and AIWC both show a
difference in Qexhaustion greater than 130% between laboratory and field groundwater.
Due to this large difference, it is helpful to further investigate the causes for this
because it is clear that AIWC has potential to be a valuable fluoride removal material,
but only if its ability to remove fluoride effectively in the field is understood.
87
Effects of pH
The higher pH in the field groundwater (8.2) than in the laboratory (6.0) likely
accounts for some of the reduced effectiveness of the materials used in field columns.
This is important because BC and AA are known to show improved fluoride removal as
the pH decreases until pH 4, a value which is unlikely in most natural or pH adjusted
systems (Abe et al., 2004; Medellin-Castillo et al., 2007; Tang et al., 2009). Therefore,
it is expected that laboratory studies would show better fluoride removal than in the
field with influent pH values of approximately 6.0 and 8.2, respectively. While in some
cases pH adjustment would be considered in field applications to improve media
performance, this was not implemented here in an effort to evaluate a simpler system
appropriate for a remote area in Ethiopia. Decreased performance of AA in the field due
to elevated pH has been documented by other researchers (Ghorai and Pant, 2005;
Hao and Huang, 1986; Tang et al., 2009).
Thus, the focus here is on evaluating the effect of pH on AIWC in the field
setting. Researchers have shown that elevated fluoride in the MER groundwater is
typically accompanied by high salinity, high pH, and low calcium content (Fawell et al.,
2006; Rango et al., 2009, 2012). The point of zero charge (PZC) for wood char alone is
9.6 ± 0.4 and for AIWC is 5.43 ± 0.25, showing a marked reduction in the PZC with the
incorporation of aluminum into the material. The lower PZC of the AIWC suggests that
this material should exhibit the best removal of negatively charged fluoride at system
pH values below 5.43 and remove less fluoride as the system pH increases above the
PZC.
88
89
key.
fluoride concentration in mg/L. Influent fluoride concentration was 8.6 mg/L and pH and competing ions vary as stated in the
Figure 4.2: Results of continuous flow columns studies where the x-axis shows bed volumes and the y-axis shows effluent
Results of a column test assessing the fluoride removal capacity of the AIWC in a lab
setting with the pH adjusted to 8.2 are shown in Figure 4.2. These results show that 78
versus 253 bed volumes passed prior to reaching Ceff = 1.5 mg/L for the field column
with 8.2 pH influent as compared with unadjusted pH influent (approximately 6.0) and
that the Qexhaustion values were 3.28 and 8.3 mg/g, respectively. This confirms batch test
results leading to the conclusion that the AIWC removes fluoride more effectively at
lower pH values and thus pH should be taken into consideration when setting up field
treatment systems.
Sulfate Competition
Another potential cause of reduced fluoride removal capacity for AIWC is the
effect of competing ions in the groundwater that were not present in the synthetic water.
While elevated levels of phosphate are often found in conjunction with elevated levels
of fluoride, in the case of MER, sulfate is the prevalent ion available that may provide
competition for fluoride removal. A preliminary test from the field site suggested that
the well water contained: sulfate = 60, phosphorous = 0.1, sodium = 490, potassium =
12 and calcium = 20 mg/L. These data are comparable to data of major ions in the MER
waters presented in Rango et al. (2010), which showed that in samples from 24 MER
groundwater well sources, the sulfate concentration ranged from below the detectable
limit to 456 mg/L, the calcium concentration averaged 21.8 mg/L, and phosphate was
not included in the data set of major ions. Table 4.3 shows the results of testing sulfate
competition with BC and AIWC. Data on this table demonstrate, with 95% confidence,
that there is no competition between sulfate and fluoride for removal by either BC or
90
AIWC for sulfate concentrations as high as 400 mg/L (4.2 mmol/L). However, it is
difficult to make a final determination based on batch tests because the fluoride
molarity in the vicinity of Qm is 5.26 mmol, while in the column studies the fluoride
was only 0.45 mmol. It is likely that at lower concentrations of fluoride there is more
potential for sulfate to compete with fluoride for adsorption and thus removal.
Therefore, a value of 250 mg/L sulfate (2.6 mmol), based on the EPA recommended
limit, was selected to test in a column setting with BC and AIWC (U.S. EPA, 2012).
Results of these tests can be observed in Figure 4.2. Comparing data from Figures 4.1
and 4.2 and Table 4.2 shows that approximate bed volumes passed to reach 1.5 mg/L
fluoride were 142 and 100 for BC with and without 250 mg/L sulfate, respectively. The
data also show that bed volumes passed to reach 1.5 mg/L fluoride for AIWC were 88
with 250 mg/L sulfate and 253 without. These results confirm the suggestion by batch
tests that BC experiences minimal effects from the presence of sulfate in solution while
the AIWC experienced a significant effect. Use of these materials in other locations
where ions such as phosphate are high or the pH is low, would warrant additional
testing to assess the fluoride removal capacity based on those changes.
Natural Organic Material
A further explanation for the reduction in the effectiveness of media in the field
versus the laboratory could be the presence of NOM in the field groundwater. NOM is
known to be widely occurring in groundwater sources in concentrations of 0.5 – 10
mg/L organic carbon (Genz et al., 2008), though it does not appear that competition in a
field setting with fluoride adsorption has yet been investigated. The literature discussing
91
water in the Main Ethiopia Rift did not yield specific types or concentrations of NOM,
therefore, suggesting this is an area helpful for researchers to assess.
However,
Newcombe et al. (1997a) stated that NOM is present in all groundwater, which would
include groundwater in the MER. Therefore, to assess the practical usefulness of these
materials for fluoride removal it is helpful to determine whether or not NOM actually
demonstrates competition with fluoride for these specific adsorptive materials. For this
work, two NOM types were selected that are representative of a range of NOM based
on molecular weight, functional groups and chemical composition. The functional
groups of these two NOM types vary as Pahokee Peat humic acid (PPHA) contains 9.01
carboxylic meq/g of carbon as opposed to the 11.61 contained by the Nordic Aquatic
Fulvic Acid (NAFA) and the PPHA contains 1.91 phenolic meq/g of carbon versus the
3.18 phenolic meq/g of carbon found in the NAFA (International Humic Substances
Society, 2013a). The chemical composition also varies as PPHA and NAFA are
partially composed of 37.34 and 45.12% oxygen and 3.69 and 0.68% nitrogen,
respectively (International Humic Substances Society, 2013b). Results in Table 4.3
show that, with 95% confidence, the NOM causes no competition for fluoride removal
with BC. NOM demonstrates minor competition with fluoride adsorption on AIWC
and ACBC as exhibited by Qm values in Table 4.3. Though no work has specifically
been done investigating reasons for NOM adsorption on ACBC or AIWC, the addition
of aluminum to the media may have an affect on NOM competition as
Qualls et al. (2009) suggested that natural organic carbon can sorb to aluminum and
thus block the pores for other ions to be removed. In this case, NOM minimally affects
fluoride removal on the media tested.
92
Table 4.3: Results of batch isotherms studying fluoride removal with and without the
presence of sulfate and natural organic material in solution.
Material
Competing
Element
Amount
(mg/L)
Amount
(mmols/L)
Langmuir
Qm (mg/g)
Batch Qe
(mg/g) at Ce=
8.6* mg/L F
Ion
Bone Char
None
NA
NA
6.1 ± 0.4
4.8
Bone Char
Sulfate
50
0.5
5.9 ± 0.4
4.8
Bone Char
Sulfate
125
1.3
6.8 ± 0.6
4.6
Bone Char
Sulfate
400
4.2
6.9 ± 0.5
4.0
Al Impregnated Wood Char
None
NA
NA
16.8 ± 5.8
12.9
Al Impregnated Wood char
Sulfate
400
4.2
14.2 ± 3.3
7.6
NOM
Bone Char
None
NA
NA
6.1 ± 0.4
4.8
Bone Char
Nordic Aquatic
10
NA
5.8 ± 0.4
4.1
Bone Char
Pahokee Peat
10
NA
5.4 ± 0.5
3.8
Bone Char
Pahokee Peat
30
NA
6.1 ± 0.5
3.8
Al Impregnated Wood Char
None
NA
NA
16.8 ± 5.8
12.9
Al Impregnated Wood Char
Nordic Aquatic
10
NA
10.5 ± 0.8
10.2
Al Impregnated Wood Char
Pahokee Peat
10
NA
9.6 ± 0.8
9.3
Al Coated Bone Char
None
NA
NA
11.4 ± 0.9
3.5
Al Coated Bone Char
Nordic Aquatic
10
NA
7.1 ± 0.8
6.3
Al Coated Bone Char
Pahokee Peat
10
NA
8.6 ± 0.7
7.0
Note: For all results shown here tests were conducted using deionized water with the competing ion/NOM added and no additional buffering or
solution manipulation was conducted.
*8.6 mg/L fluoride = 0.45 mmol/L. Thus, at 400 mg/L sulfate (nearly 10 moles of sulfate per mole of fluoride) competition, if present, would certainly
be evidenced.
Rapid Scale Small Column Test – Scaling up to Field Scale
Figure 4.3 shows the breakthrough curves of columns that had different column
volumes and particle sizes determined based on RSSCT concepts, using Equation 4.1.
The results show that the breakthrough curves overlap, even though the large-scale
column volume is nearly 5 times larger and the particle size is nearly twice as large.
93
94
1.2 ml/min.
μm , for large and small scale respectively. Starting concentration and flow rate were equivalent for both at 8.6 mg/L fluoride and
effluent fluoride concentration in mg/L. Columns have volumes of 34.9 and 7.9 cm3 and average particle sizes of 637 and 302
Figure 4.3: Graph shows results of rapid small scale column tests where the x-axis shows bed volumes and the y-axis shows
This suggests that it is possible to scale up fluoride removal using BC for a community
scale system using RSSCT concepts, i.e. the data presented here could be scaled up for
a larger scale field system at a community site. While this technique was developed for
activated carbon (Crittenden et al., 1986), and Westerhoff et al. (2005) extended the
validity of the RSSCT model for arsenic adsorption onto granular ferric hydroxide,
based on literature review this is the first work to test and offer early stage validation of
BC and fluoride. Future research should further validate the use of RSSCT equations for
BC through repeat column tests and use of additional column and particle sizes
(Westerhoff et al., 2005).
Conclusion
BC, ACBC and AIWC all proved to be effective fluoride adsorption technologies
in batch tests, laboratory column studies and field studies conducted in the Ethiopian
Rift Valley. The laboratory tests showed that each material removed fluoride more
effectively than the often tested AA, and that the removal efficiency was AIWC >
ACBC > BC. However, the field study resulted in the AIWC effectiveness being
approximately 130% lower than in the laboratory columns. Additional batch and
column studies suggest that this decrease in removal effectiveness was due to the
presence of sulfate ions and the alkaline pH (8.2) in the field groundwater. This study
also showed that two types of NOM, PPHA and NAFA, provided minimal competition
with fluoride removal on the three novel media. A final piece of this study provided
evidence that RSSCT principles can be applied to fluoride removal from water using
bone char as the sorbent. Beyond this research, additional work will be helpful to
95
assess options to improve media effectiveness, costs, local availability of materials and
user acceptance of various technologies in the Main Ethiopian Rift.
Acknowledgements
The authors would like to thank Catholic Relief Services, the Meki Catholic
Secretariat, CARE Ethiopia, and Addis Ababa University, organizations without which
the work in Ethiopia could not have occurred. The authors gratefully acknowledge the
U.S. Environmental Protection Agency and the U.S. National Science Foundation that
contributed partial funding to this work. This publication was developed under STAR
Fellowship Assistance Agreement no. 91731301 awarded by the U.S. Environmental
Protection Agency (EPA). This work has not been formally reviewed by EPA, and the
views expressed in this paper are solely those of the authors and the EPA does not
endorse any products or commercial services mentioned in this publication. The authors
also wish to acknowledge partial funding from the OU WaTER Center, the University
of Oklahoma and the Rotary Club of Norman, Oklahoma in support of this work.
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Water treatment: principles and design; 2ND Ed. John Wiley & Sons, Hoboken,
New Jersey.
U.S. EPA. (2012). “Drinking Water Contaminants.”
<http://water.epa.gov/drink/contaminants/index.cfm#SecondaryList>
(Apr. 19, 2012).
102
Westerhoff, P., Highfield, D., Badruzzaman, M., and Yoon, Y. (2005). “Rapid smallscale column tests for arsenate removal in iron oxide packed bed columns.” J.
Environ. Eng. 131(2), 262–271.
World Health Organization and UNICEF. (2012). Progress on drinking water and
sanitation 2012 update. New York, New York.
World Health Organization. (2011). Guidelines for drinking-water quality - 4th edition.
World Health Organization, Geneva, Switzerland.
Yadav, A. K., Kaushik, C. P., Haritash, A. K., Kansal, A., and Rani, N. (2006).
“Defluoridation of groundwater using brick power as an adsorbent.” J. Hazard.
Mater. B128, 289–293.
103
CHAPTER 5
Methods for Optimizing Activated Materials for Fluoride Removal
from Drinking Water Sources
Abstract
This paper presents results of amending eucalyptus wood char with the
following aluminum and iron materials: aluminum sulfate, aluminum chloride, iron (II)
chloride, and iron (III) nitrate. Results show that amending wood char with metal oxides
increases the ability of the wood char to remove fluoride from drinking water. Wood
char alone has a Qe1.5 value of 0.04 mg/g, whereas the Qe1.5 values of the metal amended
wood chars range from 0.14 to 2.06 mg/g. Interestingly, the iron oxide amendments
resulted in greater improvements to the fluoride removal capacity than the aluminum
oxide amendments, with Qe1.5 values of 2.06 and 0.76 mg/g for the two iron oxide
amended chars and 0.57 and 0.14 mg/g for the aluminum oxide amended chars.
Pretreating wood char with two oxidizing agents, potassium permanganate and
hydrogen peroxide, increased both the metal loading and fluoride removal capacities in
select instances. When wood char was pretreated with potassium permanganate and
amended with aluminum sulfate, the Qe
1.5
value increased from 0.14 to 0.40 mg/g and
the metal loading from 0.05 to 0.65%. However, an increase in fluoride removal and
metal loading was not observed for all pretreatments. Oxidation pretreatment of wood
char reduced the point of zero charge and, for hydrogen peroxide only, decreased the
specific surface area significantly (334 to 133 m2/g). Thus, this work shows the
104
potential for using oxidizing agents and aluminum and iron amendments with wood
char, and potentially other biochars, to improve their fluoride removal capacities.
Introduction
Fluoride in groundwater is a global concern as millions of people consume
drinking water containing fluoride at concentrations above the 1.5 mg/L limit
recommended by the World Health Organization (WHO) (World Health Organization,
2011; Jagtap et al., 2012). While fluoride is naturally occurring in the groundwater of
over 30 countries, it can also occur anthropogenically through processes such as
aluminum smelting and fertilizer production (Hem, 1985; Ayoob et al., 2008; Hudak,
2008). While in low doses fluoride helps prevent dental carries (Schamschula and
Barnes, 2981; Fawell et al., 2006), high fluoride uptake from food or water can be
harmful to human health, causing dental and skeletal fluorosis (Fawell et al., 2006;
Gazzano et al., 2010). Dental fluorosis typically causes darkening or mottling of teeth,
which can increase tooth sensitivity (Fawell et al., 2006; Tonguc et al., 2011). Skeletal
fluorosis causes bone deformities or stiffness and, thus, pain and/or decreased mobility
(Kaseva, 2006; Meenakshi and Maheshwari, 2006). In response, researchers globally
are investigating methods for fluoride removal from drinking water (Fawell et al., 2004;
Jagtap et al., 2012; Rao, 2003).
Methods frequently studied for removing fluoride from drinking water include:
precipitation, ion exchange, electrocoagulation, membrane filtration, and adsorption
(Pervov et al., 2000; Rao, 2003; Meenakshi and Maheshwari, 2006; Kamble et al.,
2007; Tchomgui-Kamga et al., 2010; Gwala et al., 2011). For rural regions in
105
developing countries it is helpful to investigate inexpensive, sustainable and locally
available options for fluoride removal. Adsorption is an attractive approach because it
can utilize gravity flow and thus, does not require electricity.
Depending on the
sorptive material, adsorption can be highly effective at removing fluoride from water to
meet the WHO drinking water standard. Frequently utilized adsorption materials for
fluoride removal include: bone char, activated alumina and clays or soils (Fawell et al.,
2006; Ayoob et al., 2008; Brunson and Sabatini, 2014a). While activated alumina is an
effective sorptive material, it is manufactured in select locations and may be
prohibitively expensive to obtain in developing regions. Additionally, activated alumina
performs best at pH values below normal groundwater (e.g., pH 5) (Fawell et al., 2006).
While bone char is currently used in several countries, including Ethiopia and Kenya,
bone char production must be done precisely or char quality is poor. Furthermore, some
communities are not amenable to using an adsorption material made from animal bones
for cultural or religious reasons. While most research evaluating clays and soils for
fluoride removal has shown them to be minimally effective, materials that contain iron
or aluminum hydroxides have often demonstrated higher removal capacities. Several
researchers have investigated the potential for using wood char as a sorbent, but without
amendment it has demonstrated poor fluoride removal ability thus far (Brunson and
Sabatini, 2014b; Abe et al., 2004).
This paper investigates the potential to make wood char a more effective
fluoride removal material by amending it with aluminum and iron oxides. Wood char is
a carbon-based material typically composed of carbon (50%), oxygen (44%), hydrogen
(6%) and trace amounts of metals (Pettersen, 1984). However, chemical composition
106
and other characteristics of wood and wood chars, such as specific surface area, vary
based on location, soil type, season, age and type of wood (James et al., 2005; Pettersen,
1984). Several studies have shown promising results for removing fluoride with
aluminum and iron-based metal oxide materials. Biswas et al. (2007) demonstrated that
an oxide made of aluminum and iron adsorbed fluoride with a Langmuir isotherm
maximum adsorption value of 17.7 milligrams of fluoride per gram of material. This
suggests an affinity between fluoride and metals such as aluminum and iron which can
be attributed to the positive charge on the surface of these metal oxides and the negative
charge of the fluoride ions in water at a neutral pH value. Additionally, TchomguiKamga et al. (2010) investigated the ability of an aluminum–iron precipitate mixture to
adsorb fluoride and the possibility of impregnating spruce wood with the precipitate
mixture through a boiling process. Their work showed a Langmuir maximum
adsorption of approximately 13.5 milligrams of fluoride per gram of material. These
research efforts point to the potential for improving the fluoride removal capacity of
wood char though metal oxide amendment.
Although previous studies have looked at different adsorption materials for
fluoride removal and others have tested aluminum and iron amendments on various
activated carbon materials, there is a lack of systematic studies of metal amended
biochars. A systematic investigation should utilize the same starting material with
multiple metal amendment processes and study fluoride removal capacity and surface
properties to promote better understanding. Another aspect of interest is the potential to
increase metal loading onto wood char by pretreating the wood char with oxidizing
agents. Pretreatment methods are intended to alter the surface chemistry of the media
107
which, according to Polovina et al. (1997), is one of the important characteristics of an
adsorbent.
According to Song et al. (2010), both chemical and thermal techniques can be
used to modify activated carbon surface functional groups, which include a range of
acidic and basic groups. Thus, researchers have investigated the use of oxidizing agents
as one chemical technique to alter the surface chemistry of activated carbon-based
media. For example, Song et al. (2010) evaluated the use of oxidizing agents to alter the
ability of coconut-based activated carbon to adsorb positively charged lead from water.
Their research showed that the oxidation pretreatment process caused minimal change
to the specific surface area of the activated carbon, lowered the point of zero charge,
and resulted in increased lead removal, in some cases doubling the lead adsorption
capacity of the media. Work done by Liu et al. (2007) and Polovina et al. (1997)
showed that oxidation with boiling nitric acid increased the acidic oxygen-based
functional groups on the surface of activated carbon. Hristovski et al. (2009) tested the
ability of lignite-based activated carbon modified with iron to remove arsenic. Two
procedures were tested for adding iron to activated carbon. The first involved an
alcohol and iron mixture and the second utilized pretreatment of the activated carbon
with potassium permanganate before amending it with iron. Their results concluded that
samples pretreated with the oxidant, potassium permanganate, showed a higher
percentage of iron in the activated carbon sample and a much higher arsenic removal
capacity.
Another test, conducted by Chen et al. (2007), assessed the ability of
oxidation pretreatment to enhance iron loading onto activated carbons. They found that
pretreatment with a nitric/sulfuric acid combination resulted in the highest level of iron
108
loading onto the activated carbon. However, this oxidation procedure caused several
activated carbons, wood-based ones in particular, to experience a significant loss of
mass which made them unable to withstand the pressure and flow in a column study
(Chen et al., 2007). These results suggest that oxidation pretreatment methods have the
potential to improve metal loading onto charred or activated materials.
Thus, based on previously published work, the objectives of this research were
to: (1) assess whether metal oxide amendment of wood char results in adsorbents with
higher levels of fluoride uptake than unamended wood char, (2) determine whether
pretreatment of wood char with oxidizing agents followed by metal oxide amendment
improves the fluoride removal capacity as compared to that of metal amendment alone,
and (3) investigate what, if any, effects on surface chemistry, metal loading, and surface
area result from pretreatment and amendment with oxidizing agents and metal oxides
that can help explain the results from objectives (1) and (2). The overall goal of this
work is to assess which treatments can be applied to biochars to make them useful for
absorbing fluoride from water in communities struggling with high concentrations of
geogenic fluoride. The biochar selected for study in this work was Eucalyptus wood due
to its prevalence in Ethiopia and ability to grow quickly in a variety of conditions
(Dessie and Erkossa, 2011). To accomplish these objectives, one starting material, the
wood char prepared from Eucalyptus robusta, was treated with several metal oxide
amendment methods and two oxidation pretreatment methods. The use of a consistent
starting material allows for equitable comparison of fluoride removal capacities,
assessment of changes to the char characteristics occurring as a result of amendment
and pretreatment, and formulation of recommendations for water treatment and future
109
research in this area. This research approach advances previous work in this field in two
ways: first, most work in this area has been done using an assortment of starting
materials, thus making it difficult to draw comparative conclusions between amendment
methods, and second, previous research has not included much investigation of fluoride
removal in relation to oxidation pretreatment. Additionally, while considerable work
has evaluated amending activated carbon materials with oxidizing agents, work has not
yet been done to assess specifically the effects of treating wood char with oxidizing
agents along with metal oxide amendments.
Methods and Materials
Media Preparation
Eucalyptus robusta was obtained from Mesozoic Landscapes, Inc. in Florida and
was carbonized in a temperature controlled Thermolyne oven for 4 hours at 600 °C.
After cooling, the media was crushed and sieved to capture particle sizes between 180
and 425 µm in diameter. Crushed char was rinsed to remove fines, dried at 100 °C and
stored in sealed bags until used. Media pretreatment was achieved using two methods.
In one method 1 liter of 15% hydrogen peroxide solution was mixed with 10 grams of
wood char and heated to 90 °C in a reflux condenser for four hours (Song et al., 2010).
A second method required combining a 0.1 molar permanganate solution with char in a
ratio of 0.08 grams of char per milliliter of solution. The mixture was shaken for 15
minutes and then the char was filtered out of solution (Hristovski et al., 2009). In each
case the residual char was rinsed in a continuous flow column with deionized water
until the effluent showed a consistent pH and then media was dried and stored.
110
Amendment with Metal Oxides
The four metal amendment methods used in this research were selected due to
having the highest fluoride removal capacity out of a larger group of amendment
methods and method variations. One amendment method utilized a 500 mg/L
(approximately 0.02 molar) aluminum sulfate solution. The pH was adjusted to 3.5
using 4-Morpholineethanesulfonic acid hydrate to keep the pH steady throughout the
treatment process. The solution (417 milliliters) was combined with 25 grams of wood
char, sealed in a glass container and shaken at 200 shakes per minute for five days. The
char was then filtered out of the supernatant, dried at 100 °C, rinsed with deionized
water and dried again. Aluminum chloride was used to make a 0.5 molar solution which
was mixed at ratio of 1 gram per 25 milliliters of solution for 6 hours, then diluted in
half and shaken for an additional 18 hours prior to rinsing and drying. Another method
required preparing a 1 molar solution of iron (II) chloride that was pH adjusted to 4.2 –
4.5. The solution, 150 milliliters, was mixed with 10 grams of char for 24 hours,
filtered, rinsed and dried at 80 °C for 4 hours (Gu et al., 2005). A final method involved
mixing 25 grams of ferric nitrate nonahydrate with 150 milliliters of water (0.41 molar
solution) and adding 10 grams of char. This combination was stirred for 20 minutes and
then heated to 100 °C until dry with intermittent stirring throughout the drying process.
Once dry the material was rinsed with deionized water and dried again (Vaishya and
Gupta, 2003).
111
Surface Characteristic Assessment
Specific surface area was measured using a Quantochrome 2000E Surface Area
and Pore Size Analyzer. Samples of 0.1 – 0.3 grams were degassed under vacuum at
100 °C for 24 hours. Nitrogen adsorption was evaluated at 77° K and five adsorption
points were analyzed with the Brunauer, Emmett, Teller theory to calculate specific
surface area. The drift method, based on the work of Babic (1999) was used to estimate
the point of zero charge (pHPZC) values. Potassium nitrate solutions, 0.1 M and 0.01 M,
were prepared using deionized water and 50 mL of the solutions were placed in vials.
Nitrogen gas was bubbled through the vials before and during pH adjustment to avoid
atmospheric interferences. Adjusted pH values ranged from 3 to 12 and 0.1 molar KOH
or HNO3 were utilized to obtain this pH range. Next, 0.2 grams of material was added to
each vial and vials were shaken to allow the media to equilibrate. After 24 hours, the pH
values of the solutions were measured again and the starting pH values were graphed
against final pH values to obtain the pHPZC plateau.
Batch Adsorption Studies
Batch tests were conducted in triplicate to determine the fluoride adsorption
capacity of each material. In these experiments the amount and type of media added
was held constant (e.g., 0.5 g of media in 50 mL of solution), while the fluoride
concentrations were varied (e.g. 2.5, 5, 10, 25, 50, 100 mg/L). Batch tests were shaken
at 200 shakes per minute for 24 hours to reach equilibrium. Samples were filtered using
Whatman 11 µm filter papers prior to analysis. All batch studies were conducted at a
constant ionic strength of 0.05 molar NaCl.
112
Fluoride and Metals Quantification
Fluoride concentrations were measured using a fluoride specific electrode and
an Orion pH/ISE meter, model 710A. Fluoride samples were treated with Total Ionic
Strength Adjustment Buffer I prior to analysis to ensure equal ionic strength and pH.
Samples to be analyzed for metals were digested and then analyzed on an ICP-MS.
Data Analysis
Data were analyzed using Freundlich and Langmuir isotherms. The Freundlich
equation is shown in Equation 5.1, where Qe represents the milligrams of fluoride
removed per gram of material, Ce is the fluoride concentration at system equilibrium,
and Kf and 1/n are empirical constants reflecting relative capacity and adsorption
heterogeneity, respectively.
Qe = KfCe1/n
Equation 5.1
Qe = (CeQmb)/(1+Qmb)
Equation 5.2
Equation 5.2 shows the Langmuir equation, where Qe and Ce are the same as
above, Qm is the maximum possible fluoride removal value for that system and b is
related to adsorption affinity.
Regression analyses of data were conducted using
SigmaPlot Version 12.0 software. In this work both Qm and Qe1.5 are utilized to discuss
results. Qe1.5 is the adsorption capacity exhibited by a material when the fluoride
equilibrium concentration is 1.5 mg/L and is calculated using the Freundlich equation.
While the Qm values are interesting for research, the Qe1.5 values are more relevant based
113
on common environmental fluoride conditions and the WHO recommended fluoride
concentration (1.5 mg/L).
Results and Discussion
Impact of Metal Oxide Amendment on Fluoride Removal
A primary objective of this research was to investigate whether amending wood
char with metal oxides increases the fluoride removal capacity of the wood char.
Additionally, if metal oxides do cause an increase in fluoride removal it is helpful to
assess which metal oxide amendment method is most effective, which is possible
because all amendments were tested using on a common starting material. The data in
Figure 5.1 shows that amending wood char with metal oxides increases the fluoride
removal capacity of the wood char (the unamended wood char [WC] evidenced the
lowest Qe values). Review of the Langmuir Qm values for the metal amended chars
(Table 5.1) shows that they range from 1.62 to 8.02 mg/g, as compared to the
unamended wood char at 0.68 mg/g, thus confirming the helpful ability of metal oxide
amendments to improve fluoride removal capacity of wood char. The Qe1.5 value, the Qe
value resulting when Ce = 1.5 mg/L fluoride, of wood char alone is 0.04 mg/g, while the
values of the metal oxide amended wood chars range from 0.14 mg/g, aluminum
sulfate, to 2.06 mg/g, iron (III) nitrate. Iron (III) nitrate resulted in a fluoride removal
capacity 50 times higher than wood char alone. Aluminum sulfate is a logical
amendment material to use based on its availability in places such as Ethiopia that
experience a high prevalence of fluoride in groundwater. However, the aluminum
sulfate amendment, while improving the fluoride removal versus unamended wood
114
char, does not demonstrate as much of a fluoride removal increase as the other metal
amendments at Qe1.5 values (shown in Table 5.1). Despite the fact that aluminum-based
materials are more frequently evaluated for fluoride removal from drinking water than
iron-based materials, this work demonstrates that iron amendments are also a viable
option.
9
8
Iron (III) Nitrate WC
Qe (mg/g)
7
Iron (II) Chloride WC
6
Aluminum Chloride WC
5
Aluminum Sulfate WC
Wood Char (WC)
4
3
2
1
0
0
20
40
60
80
100
Ce (mg/g)
Figure 5.1: Results of batch isotherms showing the fluoride removal capacities of
untreated wood char and metal oxide amended wood chars. Qe, the mg of fluoride
removed per gram of media is plotted against Ce, the equilibrium concentration of
fluoride in mg/L.
115
Table 5.1: Freundlich and Langmuir Constants and Qe1.5 values obtained using batch
isotherm data for wood char alone and metal oxide amended wood chars.
Freundlich and Langmuir Constants of Wood Char and Metal Oxide Amended Wood Char
Pretreatment
Metal Amendment
Kf
(mg/g)(mg/L)1/n
1/n
Qe (mg/g) at
Ce=1.5 mg/L F
(Freundlich)
Langmuir
Qmax (mg/g)
None
None
0.03 ± 0.01
0.60
0.04
0.68 ± 0.18
None
Iron (III) Nitrate
1.73 ± 0.41
0.43
2.06
8.02 ± 0.49
None
Iron (II) Chloride
0.68 ± 0.10
0.27
0.76
1.83 ± 0.23
None
Aluminum Chloride
0.51 ± 0.06
0.28
0.57
1.62 ± 0.09
None
Aluminum Sulfate
0.12 ± 0.03
0.51
0.14
2.04 ± 1.13
This is consistent with the fact that fluoride is considered a hard base, and as such, has a
strong attraction to multivalent metals, which include both iron and aluminum (Wu et
al., 2007). Given the minimal exploration of using iron for fluoride removal, the results
in Table 5.1 are particularly exciting for the field of fluoride removal from drinking
water and should be further explored both in laboratory and field tests.
Effect of Pretreatment with Oxidizing Agents on Fluoride Removal
The second objective of this research was to assess the fluoride removal results
when wood char was pretreated with oxidizing agents prior to metal oxide amendment.
Results of these studies are shown in Figure 5.2 and Table 5.2. Figure 5.2 shows that,
in some cases, pretreating the wood char with oxidizing agents increased the fluoride
removal capacity. For example, in Figure 5.2A, the wood char that was pretreated with
potassium permanganate (KM) and then amended with aluminum sulfate (KM –
Aluminum Sulfate WC) shows a higher fluoride removal capacity than metal oxide
amended wood char with no pretreatment (Aluminum Sulfate WC). The Qe1.5 value
116
(Table 5.2) for aluminum sulfate amended wood char pretreated with potassium
permanganate was 0.40 mg/g, while the aluminum sulfate with no pretreatment showed
0.14 mg/g. In contrast, Figure 5.2A and Table 5.2 show that hydrogen peroxide (HP)
pretreatment combined with aluminum sulfate amendment had a calculated Qe1.5 value
of 0, lower even than the wood char alone at 0.04 mg/g. Figure 5.2B shows a similar
trend for potassium permanganate pretreatment followed by iron (III) nitrate
amendment when higher equilibrium concentrations are compared (Langmuir Qm values
of 10.6 and 8.02 mg/g for pretreated and not pretreated, respectively).
A
4
KM - Aluminum Sulfate WC
3.5
HP - Aluminum Sulfate WC
Aluminum Sulfate WC
3
Wood Char Alone (WC)
Qe (mg/g)
2.5
2
1.5
1
0.5
0
0
20
40
60
Ce (mg/L)
117
80
100
B
9
Qe (mg/g)
8
7
KM - Iron (III) Nitrate WC
6
HP - Iron (III) Nitrate WC
5
Iron (III) Nitrate WC
4
Wood Char Alone (WC)
3
2
1
0
0
20
40
60
80
100
Ce (mg/L)
Figure 5.2: Results of batch isotherms showing the fluoride removal capacities of
untreated wood char and metal oxide amended wood chars with and without
pretreatment (KM = potassium permanganate, HP = hydrogen peroxide). Figure 5.2A
shows results of wood char treated with aluminum sulfate and Figure 5.2B shows
results of wood char treated with iron (III) chloride. Qe, the mg of fluoride removed per
gram of media is plotted against Ce, the equilibrium concentration of fluoride in mg/L.
However, the drinking water relevant Qe1.5 values show a different trend.
At an
equilibrium concentration of 1.5 mg/L, iron (III) nitrate amendment with no
pretreatment results in a Qe1.5 value of 2.06 versus 0.69 mg/g for potassium
permanganate pretreated. Again, with iron (III) nitrate amendment, the hydrogen
peroxide pretreatment lowered the fluoride removal capacity (Table 5.2).
118
Table 5.2: Freundlich and Langmuir constants obtained using batch isotherm data for
untreated, amended and pretreated wood chars.
Treated and Untreated Wood and Freundlich and Langmuir Constants
Pretreatment
None
Hydrogen
Peroxide
Potassium
Permanganate
None
Hydrogen
Peroxide
Potassium
Permanganate
None
Hydrogen
Peroxide
Potassium
Permanganate
None
Hydrogen
Peroxide
Potassium
Permanganate
None
Hydrogen
Peroxide
Potassium
Permanganate
Kf
(mg/g)(mg/L)1/n
1/n
Qe (mg/g) at
Langmuir
Ce=1.5 mg/L F
Qm (mg/g)
(Freundlich)
None
0.03 ± 0.01
0.60
0.04
0.68 ± 0.18
None
0.01 ± 0.01
0.53
0.02
0.21 ± 0.07
None
0.03 ± 0.01
0.44
0.03
0.21 ± 0.04
Aluminum Sulfate
0.12 ± 0.03
0.51
0.14
2.04 ± 1.13
Aluminum Sulfate
0.00 ± 0.00
2.12
0.00
0.00 ± 0.00
Aluminum Sulfate
0.33 ± 0.08
0.46
0.40
3.04 ± 0.80
Aluminum Chloride
0.51 ± 0.06
0.28
0.57
1.62 ± 0.09
Aluminum Chloride
0.12 ± 0.02
0.82
0.17
11.1 ± 1.12
Aluminum Chloride
0.08 ± 0.09
0.63
0.27
1.57 ± 0.10
Iron (III) Nitrate
1.73 ± 0.41
0.43
2.06
8.02 ± 0.49
Iron (III) Nitrate
1.49 ± 0.26
0.40
1.76
6.84 ± 0.17
Iron (III) Nitrate
0.55 ± 0.46
0.55
0.69
10.6 ± 1.45
Iron (II) Chloride
0.67 ± 0.10
0.28
0.75
1.86 ± 0.25
Iron (II) Chloride
0.11 ± 0.02
0.42
0.13
078 ± 0.06
Iron (II) Chloride
0.78 ± 0.09
0.19
0.85
1.61 ± 0.06
Metal Amendment
Further study of Table 5.2 identifies additional interesting points. Oxidation
pretreatment without metal amendment decreased the fluoride removal capacity of
wood char. For example, hydrogen peroxide and potassium permanganate pretreated
chars with no metal amendment both have a fluoride removal capacity (Qm = 0.21
119
mg/g) below that of the wood char without pretreatment or amendment (Qm = 0.68
mg/g). Comparing the Qe1.5 values in Table 5.2 shows that oxidizing agents have
different effects depending on the metal oxide amendment with which they are paired.
As discussed above, pretreatment with potassium permanganate resulted in higher
fluoride removal when combined with aluminum sulfate than when aluminum sulfate
amendment alone is used with wood char (Figure 5.2A and Table 5.2). On the other
hand, aluminum chloride amendment with no pretreatment exhibits a Qe1.5 value of 0.57
mg/g, whereas when a pretreatment step with potassium permanganate is added the
results show a decrease in fluoride removal to 0.27 mg/g (Table 5.2). Similar trends are
visible with the Qe1.5 values shown in Table 5.2 for iron (II) chloride and iron (III)
nitrate amended wood chars. Iron (II) chloride combined with potassium permanganate
pretreatment resulted in an increase in fluoride removal while iron (III) nitrate
combined with either oxidizing agent resulted in a lower Qe1.5 values.
The Langmuir Qm values offered in Table 5.2 offer additional insights into the
fluoride removal capacity of the materials at higher equilibrium values. For example,
the aluminum chloride amendment with no pretreatment exhibits a Qm of 1.62 mg/g,
whereas the wood char pretreated with hydrogen peroxide exhibits a Qm of 11.13 mg/g
(Table 5.2). This distinction between the Qe1.5 values and the Langmuir Qm values is
important because the Qe1.5 values reveal what is occurring at the WHO recommended
drinking water fluoride guideline, whereas the Langmuir Qm values suggest the highest
amount of fluoride that can be removed by a specific material at higher fluoride water
concentrations (potentially concentrations one to two orders of magnitude higher than
the WHO guideline).
Therefore, looking at the Qe1.5 values is a more pragmatic
120
indicator of what will be useful in a field setting to remove fluoride from drinking water
in a community, while the Langmuir Qm values are valuable from a research perspective
to understand what might be possible with various material amendments
Given the goal of finding materials that offer high fluoride removal capacities
for use in developing communities, it is helpful to compare the fluoride removal
capacities of these amended materials with commonly used adsorption media, such as
activated alumina and bone char. The work of Brunson and Sabatini (2014) offered Qe1.5
values for activated aluminum and bone char of 0.5 and 2.1 mg/g, respectively. The
data in table 5.2 show that many of the metal oxide amended and pretreated wood chars
exhibit Qe1.5 values greater than that of the activated alumina, such as the iron (II)
chloride and potassium permanganate pretreated iron (II) chloride. While the metal
amendments clearly improve the effectiveness of the unamended wood char, only the
wood char amended with iron (III) nitrate (2.06 mg/g) approaches that of the bone char
at the environmentally relevant Qe1.5 value (Table 5.2). Thus, the metal oxide amended
wood chars, particularly the iron (III) nitrate amended char, are helpful alternatives for
communities not amenable to using bone char. It is also helpful to use something like
wood char to provide a matrix for the metal oxides to adhere to so that water filtration is
possible without filter clogging a filter (e.g., clogging would occur colloidal size metal
particles).
121
Modifications caused by Metal Loading and Pretreatment with Oxidizing Agents
Metal Loading
The third objective of this work was to look for fundamental changes in surface
properties (e.g. metal content, point of zero charge and specific surface area) that could
account for the trends discussed above. Figure 5.3A shows that for all the metal oxide
amendments except aluminum chloride, higher metal loading resulted in higher Qe1.5
values. Looking first at metal amended wood chars with no pretreatment, the highest
fluoride removal capacity clearly corresponds to the highest metal loading for iron (III)
chloride with a Qe1.5 value of 2.06 mg/g and a metal loading rate of 15.8%, both of
which are much higher than wood char amended with other metals (ranging from 0.01
to 1.42 % metal loading, shown in Table 5.3 and Figure 5.3A).
1.6
1.2
% Metla Content
1
0.8
% Metla Content
1.4
18
16
14
12
10
8
6
4
2
0
*15.8
*14.5
3
A
*1.42
*Numbers are Qe1.5
values
Metal Amended WC
KM - Metal Amended
WC
Iron (III) Nitrate
*0.76
0.6
0.4
*0.4
0.2
*0.04
*0.14
*0.57
*0.27
0
Wood Char
Aluminum Sulfate Aluminum Chloride Iron (II) Chloride
122
400
350
B
*0.04
Specific Surface Area (m2/g)
*0.14
*0.76 *1.42
*0.57
*0.27
300
Metal
Amended WC
250
KM - Metal
Amended WC
*0.4
200
*Numbers are
Qe1.5 values
150
100
*0.69
*2.06
50
0
Wood Char
Aluminum
Sulfate
Aluminum
Chloride
Iron (III)
Nitrate
Iron (II)
Chloride
12
C
Metal Amended WC
10
*0.04
Point of Zero Charge pH
KM - Metal Amended WC
*Numbers are Qe1.5 values
8
*0.14 *0.4
6
*0.27
*0.76
*0.57
*0.69
4
*1.42
*2.06
2
0
Wood Char
Aluminum
Sulfate
Aluminum
Chloride
Iron (III)
Nitrate
Iron (II)
Chloride
Figure 5.3: Char characteristic analysis where 5.3A shows metal loading percentages,
5.3B shows point of zero charge values and 5.3C shows specific surface area data.
123
Table 5.3: Metal loading, specific surface area and surface chemistry characteristics of
all materials tested.
Treated and Untreated Wood Char Characteristics
Specific
Surface Area
(m2/g)
Metal Amendment
Percent
Aluminum
Percent
Iron
pHPZC
None
None
<0.01 ± 0
0.01 ± 0
9.4 ± 0.4
334 ± 5
Hydrogen Peroxide
None
<0.01 ± 0
0.01 ± 0
3.5 ± 0.2
133 ± 17
Potassium Permanganate
None
<0.01 ± 0
<0.01 ± 0
8.1 ± 0.3
300 ± 18
Pretreatment
Pretreatment
Metal Amendment
None
Aluminum Sulfate
Hydrogen Peroxide
Aluminum Sulfate
Potassium Permanganate
Aluminum Sulfate
None
Percent Aluminum
0.05 ± 0
pHPZC
Specific
Surface Area
(m2/g)
6 ± 0.6
318 ± 22
3.5 ± 0.3
116 ± 9
0.27 ± 0.01
5.9 ± 0.3
200 ± 45
Aluminum Chloride
0.01 ± 0.00
4.9 ± 0.6
302 ± 13
Hydrogen Peroxide
Aluminum Chloride
0.65 ± 0.01
4.6 ± 0.6
7±1
Potassium Permanganate
Aluminum Chloride
0.03 ± 0.00
5.4 ± 0.6
289 ± 6
Pretreatment
Metal Amendment
0.05 ± 0
Percent Iron
pHPZC
Specific
Surface Area
(m2/g)
None
Iron (III) Nitrate
15.8 ± 0.90
3.1 ± 0.3
49 ± 4
Hydrogen Peroxide
Iron (III) Nitrate
11.2 ± 0.53
3.3 ± 0.1
54 ± 9
Potassium Permanganate
Iron (III) Nitrate
14.5 ± 0.56
4.3 ± 1.3
58 ± 1
None
Iron (II) Chloride
0.59 ± 0.03
Hydrogen Peroxide
Iron (II) Chloride
0.70 ± 0.12
3.6 ± 0.1
105 ± 16
Potassium Permanganate
Iron (II) Chloride
1.25 ± 0.14
4.8 ± 0.5
313 ± 6
124
5.3 ± 0.4
319 ± 2
Additionally, iron (II) chloride and iron (III) nitrate, both with and without potassium
permanganate pretreatment, exhibit the highest metal loading rates and the highest Qe1.5
values among all the materials tested (Table 5.2). However, this trend of metal loading
correlating with fluoride removal does not hold true for the entire set of treated
materials. For example, the lowest metal loading, occurring with aluminum chloride
amendment on untreated wood char, does not correspond with the lowest Qe1.5 value,
which is instead exhibited by aluminum sulfate amendment on untreated wood char
(0.57 mg/g for aluminum chloride versus 0.14 mg/g for aluminum sulfate). Thus, while
the general trend shown by this data is that metal loading equates with fluoride removal,
there are exceptions.
Next it is helpful to assess the effect of oxidation pretreatment on metal loading.
In all cases, except for the iron (III) nitrate amended wood char, pretreatment with
potassium permanganate increased metal loading on the wood char above that of metal
amendment without pretreatment (Figure 5.3A). For example, iron (II) chloride shows a
0.59 % iron loading without pretreatment and 1.25 % iron loading when pretreated with
potassium permanganate (Table 5.3). Table 5.3 also shows that aluminum sulfate has
0.05 % aluminum loading on wood char with no pretreatment, 0.05% for the hydrogen
peroxide pretreated wood char, and 0.27% for the potassium permanganate pretreated
char. These data suggest that there is a relationship between the use of potassium
permanganate pretreatment and higher metal loading, which subsequently increases
fluoride removal capacity. However, pretreatment with hydrogen peroxide did not result
in increased metal loading or higher fluoride removal (Table 5.3). An increase in metal
loading on pretreated chars was expected due to previous research demonstrating that
125
oxidation of activated carbon increases the oxygen content on the surface of the media,
thus making it more acidic (Jia and Thomas, 2000). Jia and Thomas (2000) found a
clear link between an increase in the oxygen content of the surface of coconut-based
activated carbon and an increase in the adsorption of cadmium, a positively charged
metal ion. However, Song et al. (2010) suggested there is still much to be learned about
the methods by which activated carbons adsorb various metals. Interestingly, the metal
loading of the aluminum oxide amended materials is negligible (< 0.65 %) compared to
that of the iron oxide amendments (0.59 – 15.9 %) (Figure 5.3A and Table 5.3).
Additionally, the order of metal concentrations in the starting solutions for metal
amendment methods did not equate with the order of metal incorporated into the wood
char. Starting metal concentrations in amendment solutions went from lowest to highest
following the order of aluminum sulfate < iron (III) nitrate < aluminum chloride < iron
(II) chloride, whereas the metal loading rates on wood char went in a different order of
aluminum chloride < aluminum sulfate < iron (II) chloride < iron (III) nitrate. Thus,
starting metal concentration in solution does not necessary equate with metal loading on
the char material.
To provide more insight into the role of the metals in fluoride removal, the
removal capacities were normalized by metal content. Inspection of Figure 5.4 shows
that when the data are normalized, the aluminum chloride amended materials exhibit the
highest fluoride removal capacity per gram of metal and the iron (III) nitrate amended
materials exhibit the lowest capacity. This demonstrates that while iron (III) nitrate
offers the highest overall fluoride adsorption based on its high iron loading, when
normalized by metal loading it has the lowest adsorption efficiency. This suggests that
126
only a fraction of the iron actually participates in fluoride adsorption, likely the exterior
portion of the solid iron oxide. Specifically looking at the aluminum chloride amended
chars, the hydrogen peroxide pretreated wood char (HP – Aluminum Chloride WC)
exhibits the lowest fluoride removal effectiveness when normalized by aluminum
content, while the aluminum chloride wood char with no pretreatment (Aluminum
Chloride WC) proves to be the most effective (Figure 5.4B). This is consistent with
Figure 5.3A which shows that hydrogen peroxide aluminum chloride amended char had
the lowest aluminum loading, but with a Qe1.5 value higher than several other tested
materials.
A
30
Qe/% Aluminum
(mg Fluoride/g Aluminum)
25
20
Aluminum Sulfate WC
KM - Aluminum Sulfate WC
15
HP - Aluminum Sulfate WC
10
5
0
0
20
40
60
Ce (mg/L )
127
80
100
B
200
180
160
Qe/% Aluminum
(mg Fluoride/g Aluminum)
140
Aluminum Chloride WC
120
KM - Aluminum Chloride WC
100
HP - Aluminum Chloride WC
80
60
40
20
0
0
20
40
60
80
100
Ce (mg/L)
0.6
C
0.5
Iron (III) Nitrate WC
KM - Iron (III) Nitrate WC
HP - Iron (III) Nitrate WC
0.6
0.5
0.3
Qe/% Iron
(mg Fluoride/g Iron)
Qe/% Iron
(mg Fluoride/g Iron)
0.4
0.2
0.1
0.4
0.3
0.2
0.1
0
0
1
2
0
0
10
20
30
Ce (mg/L)
128
40
3
Ce
50
4
5
6
D
5
Iron (II) Chloride WC
Qe/% Iron
(mg Fluoride/g Iron
4
KM - Iron (II) Chloride WC
3
HP - Iron (II) Chloride WC
2
1
0
0
20
40
60
80
100
Ce (mg/g)
Figure 5.4: Data shows the fluoride removal capacities of untreated wood char and
metal oxide amended wood chars (WC), with and without pretreatment, (KM =
potassium permanganate, HP = hydrogen peroxide) normalized by metal content. The
y-axis shows Qe/metal %, the mg of fluoride removed per gram of media normalized by
metal content, and is plotted against Ce, the equilibrium concentration of fluoride in
mg/L. 5.4A shows results of wood char treated with aluminum sulfate, 5.4B shows
wood char treated with aluminum chloride, 5.4C shows wood char treated with iron
(III) nitrate with the inset graph showing data only up to Ce = 5 and 5.4D shows wood
char treated with iron (II) chloride.
When normalized by metal loading, the aluminum chloride treated char performed best
overall, which suggests there is potential to greatly improve fluoride removal capacity if
a method is found that will increase aluminum chloride loading onto char materials.
129
Tests showed that neither increasing the starting aluminum chloride concentration or
increasing the contact time between the starting aluminum chloride solution and the
wood char increased fluoride removal. Other methods, such as temperature adjustment,
method of shaking, and starting amendment solution pH, should be investigated in
future research to increase metal loading. Future work should also be done with the
aluminum based materials prior to using them in a field setting to ensure that only an
acceptable level of residual aluminum is leaching into the drinking water supply during
filtration due to the potential causative link between aluminum consumption and
Alzheimer’s disease (Walton, 2006).
Surface Chemistry
Another important media characteristic to assess is the pHPZC (solution pH at
which the surface of a material exhibits a net neutral charge). The pHPZC can depict
changes in the acidic and basic functional groups on the surface (Tchomgui-Kamga et
al., 2010). The starting pHPZC of the eucalyptus wood char was 9.4 and, as shown at the
top of Table 5.3, in each case of oxidation pretreatment the pHPZC decreased, to 8.1 for
the potassium permanganate pretreatment and 3.5 for hydrogen peroxide. It was
hypothesized that pretreating wood char with oxidizing agents would make the material
surface more acidic through the oxidation of surface functional groups. The decrease in
pHPZC exhibited by the pretreated wood char affirms this hypothesis. A lower pHPZC
means that there is a much wider range (all solution pH values above the pHPZC) in
which the surface of the material will have a negative charge, which should be more
effective for attracting positively charged metal oxides to the wood char.
130
The metal amendments also had an effect on the surface chemistry of the wood
char. Figure 5.3B and Table 5.3 show that materials amended with iron have pHPZC
values ranging from 3.1 to 5.3 and those amended with aluminum range from 3.5 to 5.9.
These are similar to results found by Tchomgui-Kamga et al. (2010) who obtained
pHPZC values ranging from 2.8 to 4.4 for their combined aluminum and iron
impregnated spruce wood, depending on the charring temperature. There are aluminum
oxides that have pHPZC values of 4.5 to 9.5 and iron oxides that range from 6.3 to 9.8;
values are variable based on crystalline structure and type of oxide (Sposito, 1995;
Cornell and Schwertmann, 2003).
Most of the aluminum amended materials fall in the range of the expected
aluminum oxide pHPZC values, such as 6.0 for aluminum sulfate amended wood char,
whereas, the iron (III) nitrate pHPZC was lower than expected, at 3.1 (Table 5.3). It is
possible that this occured because iron is a transition metal, which makes it a good
oxidant and therefore, it may have served as an oxidizing agent for the wood char. If
that is the case, the iron (III) nitrate would have added extra acidic functional groups to
the surface of the char, thus further decreasing the pHPZC. In the case of fluoride
adsorption, ideally a material that exhibits a high pHPZC value is preferred; a high pHPZC
leaves the surface of the sorbent positively charged at typical ground water pH values.
For these experiments, the prepared fluoride solutions had starting pH values of 5.5 – 7
and ending equilibrium pH values varied based on the adsorbent used. Therefore, it is
expected that any material with a pHPZC value above 6 should perform better than those
with lower pHPZC values. However, in this work, the best performing materials, iron
(III) nitrate and iron (II) chloride, exhibited pHPZC values less than 6.0 (Table 5.3).
131
These unexpected results may be the outcome of ion exchange between the hydroxide
ions, which are included in the metal oxides, and fluoride ions rather than the result of
the amended wood char surface attracting the fluoride ions from water through
electrostatic forces.
When investigating the effect of oxidation pretreatment it is difficult to see
consistent trends in pHPZC values. However, Figure 5.3B shows that, when comparing
the potassium permanganate pretreated with the metal amendment without pretreatment,
the metal treatment methods that exhibit the lowest pHPZC values also offer the highest
fluoride removal capacities. For example, the pHPZC of aluminum chloride treated wood
char with no pretreatment is 4.9 and for potassium permanganate pretreated is 5.4. The
aluminum chloride with no pretreatment has the higher Qe1.5 value of 0.57 versus 0.27
for the potassium permanganate pretreated. Future research should evaluate specific
surface functional group changes on the wood char caused by the pretreatment and
metal amendment methods to gain a better understanding of the fundamental processes
occurring during treatment.
Surface Area
A final key media characteristic is specific surface area, which showed limited
change through amendment and pretreatment in this research (Figure 5.3C and Table
5.3). The largest pretreatment change in specific surface area occurred when
pretreatment was done with hydrogen peroxide, where the specific surface area
decreased from 334 to 133 m2/g (Table 5.3). This change is expected as specific surface
area can decrease after oxidation pretreatment when strong oxidizing agents are used
132
that can cause damage to the pore walls of an activated carbon material (Ania et al.,
2002; Song et al., 2010). For nitric acid pretreatment, another common oxidizing agent
used with activated carbon, Ania et al. (2002) found a significant decrease in the
specific surface area with the use of nitric acid at 20% concentration or greater on a
commercially available activated carbon. The wood char used in this study was found to
experience a decrease in specific surface area even with just 2% nitric acid
pretreatment, which suggests that Eucalpytus robusta char may be more friable than
other activated carbon materials. The potassium permanganate exhibited minimal
change in specific surface area (300 and 334 m2/g for potassium permanganate
pretreated and untreated wood char, respectively).
Treatment with metals in most cases did not greatly alter the specific surface
area. For example, treatment with iron (II) chloride resulted in a specific surface area of
318 m2/g compared with 334 m2/g for unamended wood char (Table 5.3). However, in
other cases metal amendment significantly reduced the surface area. For example,
treatment with iron (III) nitrate reduced the specific surface area of all pretreated and
untreated wood chars from a range of 133 to 334 m2/g to between 47 and 59 m2/g. Iron
(III) nitrate also exhibited a much larger metal loading rate than any of the other
amended materials, which suggests specific surface area was lost as a larger amount of
metal was incorporated into the media. Metal amendments are expected to reduce the
specific surface area, depending on metal loading, due to metal oxides blocking some of
the internal pores. However, as shown in Figure 5.3C, the loss of surface area was not
detrimental to the fluoride removal capacity of the iron (III) nitrate, as compared with
all other treated media in this work.
133
Based on this large loss of specific surface area, it is interesting to see results
when Qe is normalized by both specific surface area and metal content. Figure 5.5
shows that when this double normalization is evaluated, the aluminum chloride
amended wood char outperforms all the other tested media, suggesting that the best use
of future research may be to find ways to enhance aluminum chloride amendment onto
wood char for fluoride removal.
Qe/SSA/Metals% [(mg fluoride*g char)/(g metal*m2)]
0.6
0.5
0.4
Aluminum Chloride WC
Aluminum Sulfate WC
0.3
Iron (II) Chloride WC
0.2
Iron (III) Nitrate WC
0.1
0
0
20
40
60
80
100
Ce (mg/g)
Figure 5.5: Results of batch isotherms showing the fluoride removal capacities of
untreated wood char and metal oxide amended wood chars normalized by both specific
surface area and metals content. Qe, the mg of fluoride removed per gram of media is
normalized by specific surface area and metals content [(mg fluoride * g char)/(g metal
* m2)] and plotted against Ce, the equilibrium concentration of fluoride in mg/L.
134
From a practical perspective, the aluminum chloride amendment is a simple procedure
and, assuming chemicals are readily available, this is an easy way to improve the
fluoride removal capacity of wood char.
Conclusion
This work offers several conclusions that provide new insights into increasing
the fluoride removal capacity of wood char amended with metal oxides. This research
demonstrates that aluminum oxide amendments, as well as the less studied iron oxide
amendments, are effective at increasing fluoride removal by wood char. Although all
four metal amendment methods showed statistically significant increases in fluoride
removal above that of wood char alone, the iron oxide amendments actually resulted in
higher fluoride adsorption than aluminum for all cases; the iron (III) nitrate showed a
Qe1.5 value over 50 times higher than that of wood char alone. Iron (III) nitrate
amendment exhibited the highest metal loading and the largest fluoride update, but also
demonstrated an inefficient use of iron when fluoride removal was normalized by metal
loading. This result can likely be attributed to the reduced specific surface area caused
by iron blocking the pore spaces in the wood char and preventing full use of pore space
and iron content. In contrast, the aluminum chloride treated wood char showed the
lowest metal loading but the highest efficiency when normalized by metal content and
specific surface area. In all cases, pretreatment with oxidizing agents decreased the
point of zero charge, and in most cases, potassium permanganate pretreatment increased
metal loading and fluoride removal. Additionally, wood char, when treated with
hydrogen peroxide or nitric acid (done in preliminary testing) was found to be less
135
robust during the oxidation process than activated carbon used in oxidation tests
conducted by other researchers.
Therefore, the specific surface area decreased
significantly with the stronger oxidation, pretreatment with hydrogen peroxide, but less
with the potassium permanganate pretreatment. This suggests the weaker oxidation
pretreatment is preferable for wood char. This work is one of the first to systematically
study one starting material with several oxidizing agents and metal amendment
processes. The conclusions from this work suggest that iron oxide amendment processes
should be further explored for increased fluoride removal, particularly in geographic
areas where iron amendment materials are readily available. Additionally, future work
assessing fluoride removal mechanisms and surface functional group changes and
investigating ways to enhance metal loading on wood char can be useful to the
community working on fluoride removal from drinking water.
Acknowledgements
The authors gratefully acknowledge the U.S. Environmental Protection Agency
and the U.S. National Science Foundation that contributed partial funding to this work.
This publication was developed under STAR Fellowship Assistance Agreement no.
91731301 awarded by the U.S. Environmental Protection Agency (EPA). This work has
not been formally reviewed by EPA, and the views expressed in this paper are solely
those of the authors and the EPA does not endorse any products or commercial services
mentioned in this publication. The authors also wish to acknowledge partial funding
from the University of Okahoma WaTER Center and the Graduate College at the
University of Oklahoma in support of this work
136
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CHAPTER 6
Conclusions
There is a global drinking water crisis affecting millions of people every day.
Identifying successful solutions that work in specific contexts around the world is
challenging. While a multitude of excellent organizations have been implementing
solutions to the water crisis for decades, an alarming number of past projects are not
demonstrating long-term sustainability. This dissertation began with a review of
literature related to implementation of new water points and drinking water treatment
technologies. The review provided examples of successes and failures in water scheme
implementation and discussed causes of failure. For example, when a new well is
drilled and implemented many things are needed to assure sustainability including: a
supply chain for spare parts, money to buy spare parts, a reliable repair person and
community buy-in for spending time or money for repairs. Lack of one or more of
these elements is frequently the cause of water point failure. One possible solution is to
incorporate business people into the research and implementation so they can evaluate
whether a supply chain is available, how supply and repair systems can be set up, and
develop a business model for financial viability of the water point or treatment system.
Business model viability varies greatly based on community income levels, education,
occupations, values, beliefs and more. To help understand some of these elements such
as community values and beliefs, it is useful to incorporate social scientists into
development and implementation so they can learn about the community and guide the
technology selection, community training, implementation and business model. Finally,
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engineers and scientists play an integral role in developing and improving upon
technologies, but their work should be influenced by the other fields of expertise on the
team. The major conclusion of this review is that implementation of water treatment
technologies and water access points can be more sustainable if multidisciplinary teams,
specifically researchers and/or practitioners from different fields of expertise, work
collaboratively on these implementations from the very beginning of a program to take
advantage of strengths and ideas from multiple areas. This conclusion, along with the
examples and resources provided, were offered in the hopes of assisting the water
community in improving sustainable research and implementation of water treatment
projects.
Building from this general foundation, the subsequent chapters in this
dissertation focused on a specific water treatment challenge. Fluoride removal from
water to make it safe to drink is an important area of research to assist with mitigation
of the global water crisis. Research has shown that bone char is an effective fluoride
adsorption material, due to favorable surface chemistry and high specific surface area.
However, in some areas bone char is not suitable for use for fluoride removal from
drinking water due to cultural or religious reasons. Therefore, this research, in addition
to investigating several practical implementation aspects of bone char, assessed the
potential for eucalyptus wood char to be a water treatment substitute for bone char.
Results showed that the specific surface area and point of zero charge of wood char
increased as charring temperature increased, and that the specific surface area of wood
charred at 600 °C is approximately 3 times higher than that of bone char.
However,
despite the high specific surface area and high pHPZC (9.6 at 600 °C), the wood char
143
evidenced low fluoride removal capacity at any charring temperature. This is likely due
to the unfavorable surface chemistry of eucalyptus wood char (despite the high phPZC)
as compared to bone char that is caused by the difference in chemical composition
between wood and bone. Amending eucalyptus wood char with aluminum oxides
significantly increased fluoride uptake – this material having both desirable surface area
and surface chemistry. When amended with aluminum nitrate the wood char exhibited
a statistically significant increase in fluoride removal capacity, which is attributed to the
strong electrostatic attraction between the aluminum oxide amendment and the fluoride.
While this capacity is still lower than bone char it is nonetheless a significant
improvement over untreated wood char. In an effort to further improve char materials
they were treated with reducing agents – this was hypothesized to increase the pHPZC
values through the removal of oxygen containing functional groups on the surface of the
media. However, results did not support this hypothesis and, though the fluoride
removal capacity of chars treated with reducing agents increased slightly, the change
was insufficient to make this a fruitful research area to further pursue.
Crucial to making this work practically useful was the study of selected
materials using continuous flow studies that are more representative of how materials
would be used in communities. Column studies were conducted in both the laboratory
and the field with four materials including: bone char, aluminum oxide coated bone
char, aluminum oxide impregnated wood char, and activated alumina. Results, not
surprisingly, found that all four materials performed better in the laboratory than in the
field. Further, the laboratory studies removed fluoride in the order of aluminum oxide
impregnated wood char > aluminum oxide coated bone char > bone char > activated
144
alumina.
The change between laboratory and field results for the aluminum oxide
impregnated wood char was particularly large with this material going from being the
best performing to the worst performing in the laboratory and the field, respectively.
From additional column and batch studies it was found that the high pH, 8.2, and the
high sulfate concentration in the Main Ethiopian Rift Valley groundwater were key
causes of the reduced effectiveness of the aluminum oxide impregnated wood char.
Thus, both pH and competing ions (e.g., sulfate) should be tested for and taken into
consideration when using adsorption materials for fluoride removal in new water
sources. In order to assess the realistic use of several fluoride removal materials in
communities, studies were done to assess whether natural organic matter offered any
competition for fluoride removal by adsorption. These studies found no competition
with fluoride removal on bone char at any equilibrium fluoride concentration and
minimal competition using other fluoride removal materials at environmentally relevant
fluoride concentrations (equilibrium values close to 1.5 mg/L). Another useful finding
of this work is evidence to support the use of scale up principles that allow small scale
laboratory studies to be used for design of community-scale systems through the use of
dimensionless rapid small scale column test (RSSCT) principles. This, and the fact that
bone char removal capacity can be improved by aluminum nitrate amendment to the
bone char, are helpful conclusions for communities where bone char is an accepted
treatment technology.
This research also investigated additional methods of amending wood char with
positively charged metals, iron and aluminum, to assess possibilities for getting higher
percentages of metals incorporated into the wood char and in doing so, increase the
145
fluoride removal capacity. This line of inquiry resulted in several conclusions that offer
insights into how the fluoride removal capacity of wood char can be increased by the
use of metal oxide amendments and pretreatment with oxidizing agents. Batch
experiments demonstrated that amendment with aluminum oxides and the less studied
iron oxides is an effective way to increase fluoride removal by wood char. While the
four tested metal amendment methods showed statistically significant increases in
fluoride removal when compared to that of wood char alone, the iron oxide amendments
resulted in higher fluoride adsorption than aluminum for all cases; iron (III) nitrated
exhibited a Qe1.5 value over 50 times higher than wood char alone. The iron (III) nitrate
amendment, when compared with the other three metal oxide amendments, resulted in
the highest metal loading on the wood char and the largest impact on fluoride removal.
However, it also demonstrated an inefficient use of iron during the fluoride removal
process, observed when fluoride removal capacities were normalized by metal loading.
This inefficiency can likely be attributed to large amounts of iron particles blocking the
wood char pores, thus preventing the full use of internal wood char pore space and
access to iron oxides that may have incorporated into the wood char below the surface.
In contrast, aluminum chloride amended wood char showed the lowest metal loading
percentage yet had the highest removal efficiency when removal capacities were
normalized by both metal contents and specific surface areas. These results suggest a
helpful area of future research; investigating ways to further maximize fluoride removal
capacity and efficient use of metal amendments for this purpose. Another hypothesis
tested in this dissertation was that pretreating wood char with oxidizing agents would
lower the pHPZC, thus, increasing metal loading onto the wood char due to the increased
146
electrostatic attraction between the char surface and the metal oxides. Results showed
that pretreating wood char with oxidizing agents did lower the pHPZC, and in most cases,
when the oxidizing agent was potassium permanganate, the lower pHPZC coincided with
increased metal loading. However, when hydrogen peroxide was used as the oxidizing
agent, the pHPZC experienced a larger decrease, but this did not result in increased metal
loading or improved fluoride removal in most cases. Wood char, when treated with
strong oxidizing agents, hydrogen peroxide or nitric acid, lost a large portion of specific
surface area, likely due to damage to the internal pore walls caused by strong oxidation.
This research systematically studied one starting material with several oxidizing agents
and metal amendment processes, which is helpful towards considering which area of
study has the most merit for future work. The conclusions from this work suggest that
iron oxide amendment processes should be further explored for fluoride removal,
particularly in geographic areas where iron amendment materials are readily available.
Additionally, future work assessing fluoride removal mechanisms and investigating
ways to enhance metal loading onto wood char can be useful to the community working
on fluoride removal from drinking water. Specific studies should be done to determine
the optimal way to amend wood char, or other biochars, with iron oxides to efficiently
use the iron oxides and gain maximum fluoride removal.
Additionally, surface
functional groups should be assessed for each amendment or pretreatment method to
better understand the interactions occurring at each step in order to determine the best
method (that is also feasible in developing regions) to produce highly effective fluoride
removal materials.
When combined together, the research reported in this dissertation contributes to
147
the body of knowledge surrounding technologies for removing fluoride from drinking
water.
The technologies and ideas that were evaluated focused on improving
knowledge about existing sustainable and inexpensive fluoride adsorption technologies.
This dissertation both assessed new options for fluoride removal and evaluated methods
for improving an existing one (bone char).
In particular this work offered the
following: RSSCT principles showed promise for scale up and design of bone char
systems, wood char can be effective for fluoride removal when amended with
aluminum or iron oxides, iron oxide amendments proved effective for fluoride and
should be further evaluated, and pretreatment with oxidizing agents, while not
necessarily practical in the field, is an option that can alter surface chemistry and metal
loading of wood char, potentially for the benefit of fluoride removal. While this
dissertation offered many helpful conclusions for researchers and practitioners, there is
still considerable work to be done to advance the field of fluoride removal and
alternative water supply to mitigate the issue of 200 million people around the world
consuming elevated fluoride concentrations in drinking water.
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Appendix A: Fluoride Removal by Bone and Wood Chars
Al Coated XA 600 C
Al Coated XA 500 C
5
Al Coated XA 400 C
4.5
Al Coated XA 300 C
4
Al Coated 600 EU
Qe (mg/g)
3.5
3
2.5
2
1.5
1
0.5
0
0
10
20
30
40
50
Ce (mg/L)
60
70
80
90
Figure A-1: Graph shows adsorption isotherms of fluoride removal by Ximenia
americana (XA) and Eucalyptus robusta (EU) wood chars amended with aluminum
oxides, plotting fluoride levels adsorbed by the chars (Qe, mg fluoride/g wood char)
versus equilibrated aqueous fluoride concentration (Ce, mg/L).
149
8
600 °C Al Coated Eucalyptus
7
400 °C Eucalyptus
500 °C Fish Bone Char
6
500 °C Al Coated Fish Bone Char
5
Qe (mg/g)
4
3
2
1
0
0
20
40
60
80
100
120
Ce (mg/L)
Figure A-2: Graph shows adsorption isotherm of fluoride removal by fish and bone
chars and aluminum oxide amended fish and bone chars, plotting fluoride levels
adsorbed by the chars (Qe, mg fluoride/g fish bone char) versus equilibrated aqueous
fluoride concentration (Ce, mg/L).
150
Appendix B: Competing Ions
8
7
6
Qe (mg/g)
5
4
Fish Bone Char
FBC 500 with 50 mg/L SO4
3
FBC 500 with 125 mg/L SO4
FBC 500 with 25 mg/L PO4
2
FBC 500 with 400 ppm SO4
1
0
0
10
20
Ce (mg/L)
30
40
50
Figure B-1: Adsorption isotherms of fluoride on fish bone char (FBC), with competing
ions sulfate (SO4) and phosphate (PO4) present in some systems as noted in the legend.
The y-axis shows fluoride levels adsorbed by the fish bone char (Qe, mg fluoride/g fish
bone char) and the x-axis versus equilibrated aqueous fluoride concentration (Ce, mg/L).
151
Appendix C: Iron Oxide Amendments
Table C-1: Results of preliminary iron oxide amendment tests conducted on wood char
with variations in amendment time, concentration and method.
Metal
Concentration
Amendment
Time
(hours)
Starting
Fluoride
Concentration
Fluoride
removal
(mg/g)
Iron oxide
7.5%
24
10
0.41
Iron oxide
7.5%
2
10
0.41
Iron oxide
30%
24
10
0.6
Iron oxide
30%
2
10
0.99
Iron oxide
0.5 molar
24
10
0.26
Iron oxide
1.0 molar
24
10
0.48
Iron oxide
3.0 molar
24
10
0.66
Amendment
152
Initial
Method
Source
Li et al.
(2010)
Li et al.
(2010)
Li et al.
(2010)
Li et al.
(2010)
Gu et al.
(2005)
Gu et al.
(2005)
Gu et al.
(2005)